Revenue - Condensed Consolidated Statement of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	$ 9,167	$ 9,388
Changes in operating assets and liabilities	$ (2,614)	(1,685)
As previously reported | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income		10,071
Changes in operating assets and liabilities		(2,368)
Adjustments | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income		(683)
Changes in operating assets and liabilities		$ 683