Acquisitions - Acquisition Of AFSC (Details) - USD ($) $ in Thousands			3 Months Ended								6 Months Ended	12 Months Ended
	Dec. 13, 2016	Jul. 01, 2016	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Acquisitions
Total revenues			$ 1,263,385	$ 1,292,132	$ 1,164,345	$ 1,117,022	$ 1,268,743	$ 1,190,082	$ 1,157,607	$ 980,968		$ 4,836,884	$ 4,597,400	$ 3,760,118
Segment profit												301,793	275,651	266,919
Assets acquired:
Goodwill			742,054				621,390				$ 742,054	742,054	621,390	566,106
Liabilities assumed:
Fair value of long-term contingent consideration			1,799				803				1,799	1,799	803
Fair value of short-term contingent consideration			9,354				$ 91,623				9,354	9,354	91,623
Change in present value of contingent consideration												(104)	44,257	$ 9,300
AFSC
Acquisitions
Base purchase price		$ 117,500
Restricted common stock issued												4,000
Amount of consideration paid in cash		113,500
Maximum potential contingent payments		10,000
Total revenues											88,100
Segment profit											14,500
Accrual for estimated contingent consideration		8,200
Assets acquired:
Current assets (includes $32,143 and $18,188 of accounts receivable and cash, respectively)		54,255
Accounts receivable		32,143,000
Cash		18,188,000
Property and equipment, net		945
Other assets		25
Other identified intangible assets		51,721
Goodwill		76,736
Total assets acquired		183,682
Liabilities assumed:
Current liabilities		23,690
Deferred tax liabilities		19,569
Other liabilities		64
Total liabilities assumed		43,323
Net assets acquired		140,359
Working capital receivable			3,200								3,200	3,200
Estimated undiscounted future contingent payments		9,000	9,000								9,000	9,000
Fair value of short-term contingent consideration			8,500								8,500	8,500
Change in present value of contingent consideration												300
Acquisition related costs												1,900
AFSC | Trade names
Acquisitions
Indefinite-lived intangible assets acquired		3,900
AFSC | Customer contracts
Acquisitions
Finite-live intangible assets acquired		$ 47,700
Period of amortization of intangible assets acquired		7 years
AFSC | Non-compete agreements
Acquisitions
Finite-live intangible assets acquired		$ 100
Period of amortization of intangible assets acquired		4 years
AFSC | Restricted Common Stock
Acquisitions
Restricted common stock issued		$ 4,000
Restricted common stock issued, vesting period		2 years
Veridicus
Acquisitions
Base purchase price	$ 72,500
Assets acquired:
Current assets (includes $32,143 and $18,188 of accounts receivable and cash, respectively)	8,961
Accounts receivable	5,625
Cash	1,569
Inventory	1,564
Property and equipment, net	6,323
Other assets	42
Other identified intangible assets	34,440
Goodwill	30,705
Total assets acquired	80,471
Liabilities assumed:
Current liabilities	6,083
Total liabilities assumed	6,083
Net assets acquired	74,388
Working capital receivable			$ 100								$ 100	100
Acquisition related costs	500											$ 300	$ 200
Veridicus | Trade names
Acquisitions
Indefinite-lived intangible assets acquired	$ 400
Veridicus | Trade names | Minimum
Acquisitions
Period of amortization of intangible assets acquired	1 year
Veridicus | Customer contracts
Acquisitions
Finite-live intangible assets acquired	$ 33,800
Veridicus | Customer contracts | Minimum
Acquisitions
Period of amortization of intangible assets acquired	6 years
Veridicus | Customer contracts | Maximum
Acquisitions
Period of amortization of intangible assets acquired	8 years
Veridicus | Non-compete agreements
Acquisitions
Finite-live intangible assets acquired	$ 300
Period of amortization of intangible assets acquired	5 years