Revenue Recognition - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Decrease in revenue	$ (1,384,965)	$ (1,318,220)	$ (1,233,842)
NET Services accrued contract payments	9,756	17,487
Deferred revenue	562	3,019
Noncurrent deferred revenue	963
Contract with customer, revenue recognized	3,019
Service expense	(1,284,603)	$ (1,223,627)	$ (1,131,963)
NET Services
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Decrease in revenue	(1,384,965)
Performance obligation satisfied in previous period	5,685
Adjustments due to ASU 2014-09 | Adjustments due to ASU 2014-09 | NET Services
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Decrease in revenue	15,488
Service expense	$ 15,488