Consolidated Company-Sponsored Investment Funds (Fair Value) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, NAV Expedient	$ 39,900
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	7,876	$ 8,413
Investments, NAV Expedient	40,016	40,413
Short equities	15,042	40,252
Derivatives	8,176	8,649
Total liabilities measured at fair value	47,505	67,182
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	291,196	231,752
Derivatives	813	1,224
Total assets measured at fair value	465,185	340,226
Short equities	15,042	40,252
Derivatives	916	1,092
Total liabilities measured at fair value	23,468	42,036
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	43,135	45,768
Derivatives	7,063	7,189
Total assets measured at fair value	1,057,277	946,146
Short equities	0	0
Derivatives	7,260	7,557
Total liabilities measured at fair value	7,260	7,557
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	5,028	5,023
Derivatives	0	0
Total assets measured at fair value	5,028	5,023
Short equities	0	0
Derivatives	0	0
Total liabilities measured at fair value	16,777	17,589
Company-sponsored investment funds | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, NAV Expedient	81	1,059
Total assets measured at fair value	660,903	575,873
Total liabilities measured at fair value	409,673	250,500
Company-sponsored investment funds | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	538,925	352,076
Total liabilities measured at fair value	406,841	248,467
Company-sponsored investment funds | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	119,100	216,997
Total liabilities measured at fair value	2,832	2,033
Company-sponsored investment funds | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	2,797	5,741
Total liabilities measured at fair value	0	0
VIEs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	14,422	4,997
Derivatives	2,853	2,081
VIEs | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	629,680	550,850
Derivatives	1,557	1,797
Investments, NAV Expedient	81	82
Derivatives	2,853	2,081
VIEs | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	526,097	341,830
Derivatives	70	58
Derivatives	30	48
VIEs | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	100,725	203,197
Derivatives	1,487	1,739
Derivatives	2,823	2,033
VIEs | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	2,777	5,741
Derivatives	0	0
Derivatives	0	0
VIEs | Short | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short equities	406,811	248,419
VIEs | Short | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short equities	406,811	248,419
VIEs | Short | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short equities	0	0
VIEs | Short | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short equities	0	0
VOEs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	10
Derivatives	9
VOEs | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	29,656	23,226
Derivatives	10
Investments, NAV Expedient		977
Derivatives	9
VOEs | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	12,757	10,188
Derivatives	1
Derivatives	0
VOEs | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	16,879	12,061
Derivatives	9
Derivatives	9
VOEs | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	20	$ 0
Derivatives	0
Derivatives	$ 0