Annual report pursuant to Section 13 and 15(d)

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Earnings Per Share (Details)

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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Earnings Per Share (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]                        
Net income available to stockholders $ 33,735 $ 51,430 $ 53,689 $ 40,270 $ 38,007 $ 49,651 $ 47,783 $ 31,928 $ 179,124 $ 167,369 $ 152,149 $ 137,664
Less: Dividends paid                 (122,017) (109,002) (91,755)  
Undistributed earnings for the period                 $ 57,107 $ 58,367 $ 60,394  
Basic and diluted shares outstanding:                        
Basic and diluted shares outstanding (in shares)                 217,988 218,244 218,583  
Basic earnings per share:                        
Basic (in dollars per share) $ 0.15 $ 0.24 $ 0.25 $ 0.18 $ 0.17 $ 0.23 $ 0.22 $ 0.15 $ 0.82 $ 0.77 $ 0.70 $ 0.63
Diluted earnings per share:                        
Diluted (in dollars per share) $ 0.15 $ 0.24 $ 0.25 $ 0.18 $ 0.17 $ 0.23 $ 0.22 $ 0.15 $ 0.82 $ 0.77 $ 0.70 $ 0.63
Common Stock                        
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]                        
Less: Dividends paid                 $ (120,930) $ (107,880) $ (90,631)  
Undistributed earnings for the period                 $ 56,567 $ 57,722 $ 59,611  
Basic and diluted shares outstanding:                        
Basic and diluted shares outstanding (in shares)                 215,927 215,831 215,749  
Basic earnings per share:                        
Distributed earnings (in dollars per share)                 $ 0.56 $ 0.50 $ 0.42  
Undistributed earnings (in dollars per share)                 0.26 0.27 0.28  
Basic (in dollars per share)                 0.82 0.77 0.70  
Diluted earnings per share:                        
Distributed earnings (in dollars per share)                 0.56 0.50 0.42  
Undistributed earnings (in dollars per share)                 0.26 0.27 0.28  
Diluted (in dollars per share)                 $ 0.82 $ 0.77 $ 0.70  
Participating Securities                        
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]                        
Less: Dividends paid                 $ (1,087) $ (1,122) $ (1,124)  
Undistributed earnings for the period                 $ 540 $ 645 $ 783  
Basic and diluted shares outstanding:                        
Basic and diluted shares outstanding (in shares)                 2,061 2,413 2,834  
Basic earnings per share:                        
Distributed earnings (in dollars per share)                 $ 0.53 $ 0.46 $ 0.40  
Undistributed earnings (in dollars per share)                 0.26 0.27 0.28  
Basic (in dollars per share)                 0.79 0.73 0.68  
Diluted earnings per share:                        
Distributed earnings (in dollars per share)                 0.53 0.46 0.40  
Undistributed earnings (in dollars per share)                 0.26 0.27 0.28  
Diluted (in dollars per share)                 $ 0.79 $ 0.73 $ 0.68