Acquisitions - Summary of the Allocation of the Purchase Price to the Fair Value of the Assets and Liabilities Assumed (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Apr. 05, 2017	Mar. 09, 2017	Dec. 31, 2016
Assets:
Cash		$ 199,558
Fees receivable		183,717
Prepaid expenses and other current assets		53,199
Property, equipment and leasehold improvements		52,414
Goodwill	$ 3,145,046	2,376,866		$ 738,453
Finite-lived intangible assets		1,589,990
Other assets		195,041
Total assets		4,650,785
Liabilities:
Accounts payable and accrued liabilities		133,594
Deferred revenues (current)		259,672
Other liabilities		604,491
Total liabilities		997,757
Net assets acquired		3,653,028
CEB
Assets:
Cash		194,706
Fees receivable		175,440
Prepaid expenses and other current assets		52,032
Property, equipment and leasehold improvements		51,751
Goodwill		2,267,087
Finite-lived intangible assets		1,574,100
Other assets		182,720
Total assets		4,497,836
Liabilities:
Accounts payable and accrued liabilities		130,544
Deferred revenues (current)		246,472
Other liabilities		601,991
Total liabilities		979,007
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			109,779
Finite-lived intangible assets			15,890
Other assets			12,321
Total assets			152,949
Liabilities:
Accounts payable and accrued liabilities			3,050
Deferred revenues (current)			13,200
Other liabilities			2,500
Total liabilities			18,750
Net assets acquired			$ 134,199