Business and Significant Accounting Policies (Detail)	3 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) segment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Business and Significant Accounting Policies [Line Items]
Number of reportable segments | segment		3
Acquisition and integration charges		$ 42,598,000	$ 26,175,000	$ 21,867,000
Unbilled fees receivable		45,700,000	43,200,000
Stock-based compensation expense		46,661,000	46,149,000	38,845,000
Rent expense		38,000,000	33,800,000	31,500,000
Depreciation expense		37,172,000	33,789,000	31,186,000
Net capitalized development costs for internal use software		16,600,000	14,100,000
Amortization of capitalized internal software development costs		8,800,000	8,200,000	8,200,000
Amortization of intangibles		24,797,000	13,342,000	8,226,000
Pension expense		3,500,000	3,500,000	3,400,000
Foreign currency transaction gain (loss)		(400,000)	(2,600,000)	(1,700,000)
Derivative instruments, gain (loss) recognized in income, net		7,900,000	8,600,000	3,600,000
Payments for purchases of treasury stock		58,961,000	509,049,000	432,006,000
Goodwill, impairment loss	$ 0
Impairment of long-lived assets		0	0	0
Foreign currency exchange contracts
Business and Significant Accounting Policies [Line Items]
Derivative instruments, gain (loss) recognized in income, net		$ (300,000)	(100,000)	$ 600,000
Software
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		3 years
Minimum | Trade Names
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		2 years
Minimum | Customer Relationships
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		4 years
Minimum | Content
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		1 year 6 months
Minimum | Non-Compete
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		3 years
Maximum | Trade Names
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		4 years
Maximum | Customer Relationships
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		7 years
Maximum | Content
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		4 years
Maximum | Non-Compete
Business and Significant Accounting Policies [Line Items]
Remaining amortization period		5 years
Software Advice, Inc., Market-Visio Oy, and SircleIT Inc.
Business and Significant Accounting Policies [Line Items]
Decrease in contingent liability		$ 22,700,000	$ 9,200,000