Long-Lived Assets - Summary of Aggregate Amortization (Parenthetical) (Details)	12 Months Ended
	Dec. 31, 2017
Acquired client contracts | Minimum
Finite Lived Intangible Assets [Line Items]
Estimated useful life	5 years
Acquired client contracts | Maximum
Finite Lived Intangible Assets [Line Items]
Estimated useful life	10 years
Acquired software | Minimum
Finite Lived Intangible Assets [Line Items]
Estimated useful life	5 years
Acquired software | Maximum
Finite Lived Intangible Assets [Line Items]
Estimated useful life	10 years
Internal use software | Minimum
Finite Lived Intangible Assets [Line Items]
Estimated useful life	12 months
Internal use software | Maximum
Finite Lived Intangible Assets [Line Items]
Estimated useful life	10 years