Revenue - Condensed Consolidated Statement of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	$ 15,541	$ 7,290	$ 39,299	$ 32,217
Deferred income tax expense			3,503	3,822
Changes in operating assets and liabilities			$ (3,279)	1,025
ASU 2014-09 | As previously reported
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income		7,537		33,245
Deferred income tax expense				3,919
Changes in operating assets and liabilities				(100)
ASU 2014-09 | Adjustments
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income		$ (247)		(1,028)
Deferred income tax expense				(97)
Changes in operating assets and liabilities				$ 1,125