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Summary of significant accounting policies, Part 6 (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Increase (decrease) in deferred revenue $ 15,022 $ 4,602 $ 3,474
Income tax expense (benefit) $ 36,039 $ 36,697 $ 44,300
Accounting Standards Update 2016-09 [Member]      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Change to estimated effective annual tax rate   4.00%  
Income tax expense (benefit)   $ (3,300)