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, 2019	Dec. 31, 2018	Dec. 31, 2017	Apr. 05, 2017	Mar. 09, 2017
Assets:
Goodwill	$ 2,937,726	$ 2,923,136	$ 2,987,294
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