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Change in Accounting Policies - Narrative (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Asset associated with future estimated variable consideration	$ 9,200	$ 9,200
Increase (decrease) in retained earnings	$ 969,406		$ 922,662
Accounting Standards Update, 2014-09, Future Estimated Variable Consideration
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Asset associated with future estimated variable consideration		9,200
Increase (decrease) in retained earnings		6,500
Accounting Standards Update, 2014-09, Determination And Capitalization Of Direct Costs Incurred
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase (decrease) in retained earnings		(5,100)
Prepaid commissions		$ 7,200