Acquisitions (Details) - USD ($) $ in Thousands	Sep. 23, 2016	Jun. 20, 2014	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Payables recorded under contingent payment arrangements			$ 11,893		$ 0	$ 9,365
Goodwill			3,066,700		3,044,807
Redeemable non-controlling interest			$ 392,959		$ 13,203
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]
Definite-lived intangible assets acquired	$ 10,000
CPH Capital Fondsmaeglerselskab A/S [Member]
Business Acquisition [Line Items]
Ownership percentage		81.70%	90.40%
Assets under management		$ 3,000,000
Cash paid		64,400
Goodwill		58,100
Definite-lived intangible assets acquired		24,100
Net contingent consideration payable		$ 9,400
Assets under management remeasurement period		3 years
Indefinite-lived intangible assets acquired		$ 3,500
Redeemable non-controlling interest		$ 16,500