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Unearned Revenue - Changes in Unearned Revenue (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Unearned Revenue      
Balance, beginning of period $ 105,837 $ 77,022  
Billings, excluding billings for customer arrangements with termination rights 196,351 205,816  
Additions to unearned revenue upon expiration of termination rights 20,395 17,651  
Recognition of revenue, net of change in unbilled accounts receivable (204,372) (194,652)  
Balance, end of period 118,211 105,837 $ 77,022
Increase (decrease) in total unbilled accounts receivable 864 (1,460)  
Revenue Reported in Consolidated Statement of Operations $ 205,236 $ 193,192 $ 179,758