Goodwill and Identifiable Intangible Assets - Schedule of Finite-lived and Indefinite-lived Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Finite-lived and Indefinite-lived Intangible Assets [Line Items]
Accumulated Amortization	$ (341,666)	$ (293,072)
Total	348,530
Gross Carrying Amount, Trade names (unamortized)	198,739	183,239
Accumulated Impairment Charge, Trade names (unamortized)	(52,233)	(51,865)
Total, Trade names (unamortized)	146,506	131,374
Gross Carrying Amount, Total	893,769	837,169
Accumulated Impairment Charge, Total	(57,067)	(56,699)
Total	495,036	487,398
Contract backlog
Schedule of Finite-lived and Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	55,545	48,645
Accumulated Amortization	(55,229)	(48,412)
Accumulated Impairment Charge	0	0
Total	316	233
Developed technology/Vendor network
Schedule of Finite-lived and Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	95,661	95,661
Accumulated Amortization	(50,479)	(45,616)
Accumulated Impairment Charge	0	0
Total	45,182	50,045
Customer relationships
Schedule of Finite-lived and Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	500,756	466,556
Accumulated Amortization	(206,319)	(173,156)
Accumulated Impairment Charge	(4,834)	(4,834)
Total	289,603	288,566
Non-competition agreements
Schedule of Finite-lived and Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	10,220	10,220
Accumulated Amortization	(10,178)	(10,041)
Accumulated Impairment Charge	0	0
Total	42	179
Trade names (amortized)
Schedule of Finite-lived and Indefinite-lived Intangible Assets [Line Items]
Gross Carrying Amount	32,848	32,848
Accumulated Amortization	(19,461)	(15,847)
Accumulated Impairment Charge	0	0
Total	$ 13,387	$ 17,001