Consolidated Company-Sponsored Investment Funds (Fair Value) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, NAV Expedient	$ 37,600
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	5,846	$ 8,413
Investments, NAV Expedient	37,710	40,413
Short equities	20,539	40,252
Derivatives	6,600	8,649
Total liabilities measured at fair value	52,514	67,182
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	309,691	231,752
Derivatives	1,111	1,224
Total assets measured at fair value	463,695	340,226
Short equities	20,539	40,252
Derivatives	962	1,092
Total liabilities measured at fair value	34,773	42,036
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	67,699	45,768
Derivatives	4,735	7,189
Total assets measured at fair value	795,463	946,146
Short equities	0	0
Derivatives	5,638	7,557
Total liabilities measured at fair value	5,638	7,557
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,070	5,023
Derivatives	0	0
Total assets measured at fair value	1,070	5,023
Short equities	0	0
Derivatives	0	0
Total liabilities measured at fair value	12,103	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	801,751	575,873
Total liabilities measured at fair value	586,294	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	670,263	352,076
Total liabilities measured at fair value	583,602	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	129,389	216,997
Total liabilities measured at fair value	2,692	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	2,018	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	16,767	4,997
Derivatives	2,668	2,081
VIEs | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	770,953	550,850
Derivatives	1,736	1,797
Investments, NAV Expedient	81	82
Derivatives	2,668	2,081
VIEs | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	663,348	341,830
Derivatives	68	58
Derivatives	104	48
VIEs | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	105,627	203,197
Derivatives	1,668	1,739
Derivatives	2,564	2,033
VIEs | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,897	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	583,491	248,419
VIEs | Short | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short equities	583,491	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	80
Derivatives	135
VOEs | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	28,982	23,226
Derivatives	80
Investments, NAV Expedient		977
Derivatives	135
VOEs | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	6,830	10,188
Derivatives	17
Derivatives	7
VOEs | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	22,031	12,061
Derivatives	63
Derivatives	128
VOEs | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	121	$ 0
Derivatives	0
Derivatives	$ 0