Acquisitions (Details) $ in Thousands, NZD in Millions, AUD in Millions	1 Months Ended							3 Months Ended	12 Months Ended
	Aug. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Apr. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Oct. 31, 2015 USD ($)	Oct. 31, 2015 AUD	Sep. 30, 2015 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jan. 31, 2016 USD ($)	Jan. 31, 2016 NZD
Business Acquisition [Line Items]
Impairment loss on investment in affiliates									$ (3,811)	$ (23,431)	$ 0
Acquisitions									129,505	226,907
Acquisition related costs									4,500	$ 6,900	3,900
Cordell Information Pty Ltd [Member]
Business Acquisition [Line Items]
Property and equipment					$ 14,300
Acquisitions					31,900
Amount paid to acquire business					49,100	AUD 70.0
Myriad Development, Inc. [Member]
Business Acquisition [Line Items]
Consideration transferred in acquisition	$ 22,000
Contingent consideration liability									3,000
Contingent consideration change in value									300
Deferred tax liabilities assumed	3,100
Acquisitions	17,300
Clareity Ventures, Inc. [Member]
Business Acquisition [Line Items]
Consideration transferred in acquisition	15,000
Deferred tax liabilities assumed	2,600
Mercury, Inc. [Member]
Business Acquisition [Line Items]
Deferred tax liabilities assumed	19,800
Acquisitions	105,300
Adjustments recorded to goodwill								$ 5,800
FNC, Inc. [Member]
Business Acquisition [Line Items]
Consideration transferred in acquisition			$ 475,000
Contingent consideration liability			8,000
Deferred tax liabilities assumed			85,400
Acquisitions			220,300
Adjustments recorded to goodwill									5,400
Amount paid to acquire business			400,000
Other payments to acquire businesses			75,000
PIQ Acquisition [Member]
Business Acquisition [Line Items]
Remaining equity interest acquired												40.00%	40.00%
Mandatorily redeemable noncontrolling interests												$ 19,000	NZD 27.8
RELS LLC [Member]
Business Acquisition [Line Items]
Property and equipment				$ 27,000							27,000
Acquisitions				23,100
Goodwill expected tax deductible amount				11,500							$ 11,500
Amount paid to acquire business				65,000
Gain on investments and other, net				34,300
Landsafe [Member]
Business Acquisition [Line Items]
Acquisitions							$ 64,600
Amount paid to acquire business							122,000
Client lists | Cordell Information Pty Ltd [Member]
Business Acquisition [Line Items]
Intangible assets acquired					$ 5,500
Intangible assets, useful life					8 years	8 years
Client lists | Myriad Development, Inc. [Member]
Business Acquisition [Line Items]
Intangible assets acquired	$ 1,700
Intangible assets, useful life	12 years
Client lists | Clareity Ventures, Inc. [Member]
Business Acquisition [Line Items]
Intangible assets acquired	$ 3,400
Intangible assets, useful life	10 years
Acquisitions	$ 10,900
Client lists | Mercury, Inc. [Member]
Business Acquisition [Line Items]
Intangible assets acquired	$ 41,300
Intangible assets, useful life	10 years
Client lists | FNC, Inc. [Member]
Business Acquisition [Line Items]
Intangible assets acquired			$ 145,300
Intangible assets, useful life			16 years
Client lists | RELS LLC [Member]
Business Acquisition [Line Items]
Intangible assets acquired				$ 48,400
Intangible assets, useful life				10 years
Client lists | Landsafe [Member]
Business Acquisition [Line Items]
Intangible assets acquired							$ 53,400
Intangible assets, useful life							10 years
Other Intangible Assets [Member] | RELS LLC [Member]
Business Acquisition [Line Items]
Intangible assets acquired				$ 5,000
Intangible assets, useful life				10 years
Other Intangible Assets [Member] | Landsafe [Member]
Business Acquisition [Line Items]
Intangible assets acquired							$ 4,300
Intangible assets, useful life							10 years
Trade Names [Member] | Cordell Information Pty Ltd [Member]
Business Acquisition [Line Items]
Intangible assets acquired					$ 600
Intangible assets, useful life					4 years	4 years
Trade Names [Member] | Myriad Development, Inc. [Member]
Business Acquisition [Line Items]
Intangible assets acquired	$ 1,600
Intangible assets, useful life	7 years
Trade Names [Member] | Clareity Ventures, Inc. [Member]
Business Acquisition [Line Items]
Intangible assets acquired	$ 900
Intangible assets, useful life	7 years
Trade Names [Member] | Mercury, Inc. [Member]
Business Acquisition [Line Items]
Intangible assets acquired	$ 3,600
Intangible assets, useful life	8 years
Trade Names [Member] | FNC, Inc. [Member]
Business Acquisition [Line Items]
Intangible assets acquired			$ 15,900
Intangible assets, useful life			19 years
Non-compete agreements | FNC, Inc. [Member]
Business Acquisition [Line Items]
Intangible assets acquired			$ 18,800
Intangible assets, useful life			5 years
Technology-Based Intangible Assets [Member] | Myriad Development, Inc. [Member]
Business Acquisition [Line Items]
Intangible assets acquired	$ 5,800
Intangible assets, useful life	8 years
Technology-Based Intangible Assets [Member] | Clareity Ventures, Inc. [Member]
Business Acquisition [Line Items]
Intangible assets acquired	$ 2,000
Intangible assets, useful life	5 years
Adjustments recorded to goodwill								$ (200)
Technology-Based Intangible Assets [Member] | Mercury, Inc. [Member]
Business Acquisition [Line Items]
Intangible assets acquired	$ 20,100
Intangible assets, useful life	9 years
Property, Plant and Equipment [Member] | Cordell Information Pty Ltd [Member]
Business Acquisition [Line Items]
Intangible assets, useful life					10 years	10 years
Property, Plant and Equipment [Member] | FNC, Inc. [Member]
Business Acquisition [Line Items]
Property and equipment			$ 79,800
Intangible assets, useful life			12 years
Property, Plant and Equipment [Member] | RELS LLC [Member]
Business Acquisition [Line Items]
Intangible assets, useful life				10 years
Mercury, Inc. [Member]
Business Acquisition [Line Items]
Consideration transferred in acquisition	$ 83,000
Impairment loss on investment in affiliates	$ 1,900
Remaining equity interest acquired		45.00%
Payments to Acquire Equity Method Investments		$ 70,000
Voting interest acquired	55.00%
Call option, fair value		$ 4,600
RELS LLC [Member]
Business Acquisition [Line Items]
Remaining equity interest acquired				49.90%							49.90%
Ownership percentage in equity method investment				50.10%							50.10%
Fair Value | Myriad Development, Inc. [Member]
Business Acquisition [Line Items]
Contingent consideration liability									$ 1,800