Acquisitions (Details) - USD ($) $ in Thousands	Sep. 23, 2016	Jun. 20, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Payables recorded under contingent payment arrangements			$ 0	$ 11,893	$ 0
Goodwill			3,066,700	3,066,700
Redeemable non-controlling interest			$ 601,587	$ 392,959
Ramius Alternative Solutions LLC [Member]
Business Acquisition [Line Items]
Ownership percentage	100.00%
Assets under management	$ 2,500,000
Cash paid	20,500
Payables recorded under contingent payment arrangements	$ 11,900
Revenue remeasurement period	5 years
Goodwill	$ 21,900
Ramius Alternative Solutions LLC [Member] | Investment Management Contract [Member]
Business Acquisition [Line Items]
Finite-lived intangible assets acquired	$ 10,000
CPH Capital Fondsmaeglerselskab A/S [Member]
Business Acquisition [Line Items]
Ownership percentage		81.70%	93.60%
Assets under management		$ 3,000,000
Cash paid		64,400
Payables recorded under contingent payment arrangements		9,400
Goodwill		58,100
Finite-lived intangible assets acquired		$ 24,100
Assets under management remeasurement period		3 years
Indefinite-lived intangible assets acquired		$ 3,500
Redeemable non-controlling interest		$ 16,500