Changes in unearned revenue (Details) - USD ($) $ in Thousands |
9 Months Ended | 12 Months Ended | |
|---|---|---|---|
Sep. 30, 2019 |
Sep. 30, 2018 |
Dec. 31, 2018 |
|
| Balance at beginning of year | $ 127,075 | $ 117,614 | $ 117,614 |
| Deferral of unearned revenue | 141,867 | 134,527 | 166,053 |
| Recognition of unearned revenue | (122,465) | (117,297) | (156,592) |
| Balance at end of period | $ 146,477 | $ 134,844 | $ 127,075 |
| X | ||||||||||
- Definition Contract with Customer, Liability, Increase from Cash Receipts No definition available.
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| X | ||||||||||
- Definition Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable. Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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| X | ||||||||||
- Definition Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due. Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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