Investments (Summary of Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Summary of Investment Holdings [Line Items]
Investments owned	$ 763,285	$ 669,800
Long-term incentive compensation obligations	64,296	78,154
Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	385,200	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	35,999	48,761
Investments held by consolidated VIEs
Summary of Investment Holdings [Line Items]
Investments owned	247,103	0
Available-for-sale
Summary of Investment Holdings [Line Items]
Investments owned	305	364
Trading | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	47,681	59,150
Trading | U.S. Treasury Bills
Summary of Investment Holdings [Line Items]
Investments owned	29,899	24,942
Trading | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	271,787	406,322
Trading | Equities
Summary of Investment Holdings [Line Items]
Investments owned	67,610	43,584
Trading | Exchange-traded options
Summary of Investment Holdings [Line Items]
Investments owned	8,045	5,910
Investments in limited partnership hedge funds | Long-term incentive compensation-related
Summary of Investment Holdings [Line Items]
Investments owned	16,615	19,004
Investments in limited partnership hedge funds | Seed capital
Summary of Investment Holdings [Line Items]
Investments owned	20,565	20,082
Other
Summary of Investment Holdings [Line Items]
Investments owned	$ 17,676	$ 17,784