Long-Lived Assets - Summary of Aggregate Amortization (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments in client contracts
Finite Lived Intangible Assets [Line Items]
Total amortization expense	$ 7,402	$ 6,591	$ 5,165
Capitalized costs
Finite Lived Intangible Assets [Line Items]
Total amortization expense	5,141	2,755	1,334
Acquired client contracts
Finite Lived Intangible Assets [Line Items]
Total amortization expense	5,406	6,683	8,902
Client contracts
Finite Lived Intangible Assets [Line Items]
Total amortization expense	17,949	16,029	15,401
Acquired software
Finite Lived Intangible Assets [Line Items]
Total amortization expense	1,458	1,806	3,081
Internal use software
Finite Lived Intangible Assets [Line Items]
Total amortization expense	7,845	7,510	8,048
Software
Finite Lived Intangible Assets [Line Items]
Total amortization expense	$ 9,303	$ 9,316	$ 11,129