Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax [Line Items]
Non-current deferred tax assets	$ 31,067	$ 27,275
Non-current deferred tax liabilities	206,338	22,520
Valuation allowance	3,192	1,431
Tax act, provisional tax expense	123,200
Tax act, provisional amount of one-time transition tax liability	63,600
Tax act, increase in income tax expense, transition tax	63,600
Unrecognized tax benefits	60,269	37,099	$ 25,911
Unrecognized tax benefits reductions resulting from settlements with taxing authorities and lapse of applicable statute of limitations	6,300
Unrecognized tax benefits that would impact effective tax rate	57,100
Unrecognized tax benefits that would result adjustments to other tax accounts	3,200
Income tax penalties and interest expense	6,400	4,300
Income tax examination, penalties and interest expense	900	900
Deferred tax liability previously recorded in purchase accounting	12,000
Accumulated undistributed earnings of non-US subsidiaries	194,000
Accounting Standards Update 2016-09
Income Tax [Line Items]
Benefit for income taxes	12,900	10,000
CEB Talent Assessment | Disposal Group, Held-for-sale, Not Discontinued Operations
Income Tax [Line Items]
Deferred tax asset for CEB talent assessment business	62,900
State and local jurisdiction
Income Tax [Line Items]
Operating loss carryforwards	91,500
Tax credit carryforwards	$ 2,000
State and local jurisdiction | Minimum
Income Tax [Line Items]
Tax credit carryforwards, expiration period	5 years
State and local jurisdiction | Maximum
Income Tax [Line Items]
Tax credit carryforwards, expiration period	6 years
State and local jurisdiction | Expire within one to five years
Income Tax [Line Items]
Operating loss carryforwards	$ 900
State and local jurisdiction | Expire within one to five years | Minimum
Income Tax [Line Items]
Operating loss carryforwards expiration period	1 year
State and local jurisdiction | Expire within one to five years | Maximum
Income Tax [Line Items]
Operating loss carryforwards expiration period	5 years
State and local jurisdiction | Expire within six to fifteen years
Income Tax [Line Items]
Operating loss carryforwards	$ 19,900
State and local jurisdiction | Expire within six to fifteen years | Minimum
Income Tax [Line Items]
Operating loss carryforwards expiration period	6 years
State and local jurisdiction | Expire within six to fifteen years | Maximum
Income Tax [Line Items]
Operating loss carryforwards expiration period	15 years
State and local jurisdiction | Expire within sixteen to twenty years
Income Tax [Line Items]
Operating loss carryforwards	$ 70,700
State and local jurisdiction | Expire within sixteen to twenty years | Minimum
Income Tax [Line Items]
Operating loss carryforwards expiration period	16 years
State and local jurisdiction | Expire within sixteen to twenty years | Maximum
Income Tax [Line Items]
Operating loss carryforwards expiration period	20 days
Foreign Tax Authority
Income Tax [Line Items]
Operating loss carryforwards	$ 16,000
Foreign tax credit carryforwards	59,000
Foreign Tax Authority | Expire over next twenty years
Income Tax [Line Items]
Operating loss carryforwards	$ 500
Operating loss carryforwards expiration period	20 years
Foreign Tax Authority | Carried forward indefinitely
Income Tax [Line Items]
Operating loss carryforwards	$ 15,500
Other assets
Income Tax [Line Items]
Non-current deferred tax assets	30,500	27,300
Other liabilities
Income Tax [Line Items]
Non-current deferred tax liabilities	$ 253,700	$ 22,500