Investments (Summary of Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Summary of Investment Holdings [Line Items]
Investments owned	$ 772,052	$ 669,800
Long-term incentive compensation obligations	65,511	78,154
Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	395,400	478,000
Consolidated private equity fund
Summary of Investment Holdings [Line Items]
Investments owned	0	23,897
Private equity
Summary of Investment Holdings [Line Items]
Investments owned	38,248	48,761
Investments in consolidated VIEs
Summary of Investment Holdings [Line Items]
Investments owned	243,248	0
Available-for-sale
Summary of Investment Holdings [Line Items]
Investments owned	293	364
Trading | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	49,120	59,150
Trading | U.S. Treasury Bills
Summary of Investment Holdings [Line Items]
Investments owned	29,961	24,942
Trading | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	282,176	406,322
Trading | Equities
Summary of Investment Holdings [Line Items]
Investments owned	70,315	43,584
Trading | Exchange-traded options
Summary of Investment Holdings [Line Items]
Investments owned	4,709	5,910
Investments in limited partnership hedge funds | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	16,391	19,004
Investments in limited partnership hedge funds | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	19,695	20,082
Other
Summary of Investment Holdings [Line Items]
Investments owned	$ 17,896	$ 17,784