Acquisitions and Divestiture - Summary of the Allocation of the Purchase Price to the Fair Value of the Assets and Liabilities Assumed (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Apr. 05, 2017	Mar. 09, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets:
Goodwill	$ 2,987,294			$ 738,453	$ 715,359
CEB and L2 Acquisitions
Assets:
Cash	199,558
Fees receivable	183,717
Prepaid expenses and other current assets	54,777
Property, equipment and leasehold improvements	52,062
Goodwill	2,457,791
Finite-lived intangible assets	1,600,190
Other assets	79,885
Total assets	4,627,980
Liabilities:
Accounts payable and accrued liabilities	145,184
Deferred revenues (current)	259,672
Other liabilities	570,096
Total liabilities	974,952
Net assets acquired	$ 3,653,028
CEB
Assets:
Cash		$ 194,706
Fees receivable		175,440
Prepaid expenses and other current assets		53,610
Property, equipment and leasehold improvements		51,399
Goodwill		2,349,589
Finite-lived intangible assets		1,584,300
Other assets		66,818
Total assets		4,475,862
Liabilities:
Accounts payable and accrued liabilities		142,134
Deferred revenues (current)		246,472
Other liabilities		568,427
Total liabilities		957,033
Net assets acquired		$ 3,518,829
L2, Inc.
Assets:
Cash			$ 4,852
Fees receivable			8,277
Prepaid expenses and other current assets			1,167
Property, equipment and leasehold improvements			663
Goodwill			108,202
Finite-lived intangible assets			15,890
Other assets			13,067
Total assets			152,118
Liabilities:
Accounts payable and accrued liabilities			3,050
Deferred revenues (current)			13,200
Other liabilities			1,669
Total liabilities			17,919
Net assets acquired			$ 134,199