Long-Lived Assets - Summary of Carrying Value of Other Intangible Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 269,863
Accumulated Amortization	(198,047)
Net Amount	71,816
Balances without Adoption of ASC 606 | ASC 606
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount		$ 276,627
Accumulated Amortization		(206,095)
Net Amount		70,532
Investments in client contracts | Balances without Adoption of ASC 606 | ASC 606
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount		26,616
Accumulated Amortization		(9,782)
Net Amount		16,834
Capitalized costs | Balances without Adoption of ASC 606 | ASC 606
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount		26,811
Accumulated Amortization		(10,039)
Net Amount		16,772
Acquired client contracts
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	121,149
Accumulated Amortization	(81,286)	0
Net Amount	39,863
Acquired client contracts | Balances without Adoption of ASC 606 | ASC 606
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount		87,308
Accumulated Amortization		(77,288)
Net Amount		10,020
Client contracts
Finite Lived Intangible Assets [Line Items]
Accumulated Amortization	0	(97,109)
Net Amount		43,626
Client contracts | Balances without Adoption of ASC 606 | ASC 606
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount		140,735
Accumulated Amortization		(97,109)
Net Amount	66,364	43,626
Software
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount	148,714
Accumulated Amortization	(116,761)	(108,986)
Net Amount	$ 31,953	26,906
Software | Balances without Adoption of ASC 606 | ASC 606
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount		135,892
Accumulated Amortization		(108,986)
Net Amount		$ 26,906