Consolidated Company-Sponsored Investment Funds (Fair Value) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	$ 14,113	$ 11,619
Short equities	7,454	16,376
Derivatives	14,552	14,323
Total liabilities measured at fair value	51,308	55,139
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	255,053	281,736
Derivatives	2,105	948
Total assets measured at fair value	342,637	344,755
Short equities	7,454	16,376
Derivatives	985	2,540
Total liabilities measured at fair value	26,781	32,501
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	54,653	69,227
Derivatives	12,008	10,671
Total assets measured at fair value	1,355,528	896,248
Short equities	0	0
Derivatives	13,567	11,783
Total liabilities measured at fair value	13,567	11,783
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	117	1,071
Derivatives	0	0
Total assets measured at fair value	117	1,071
Short equities	0	0
Derivatives	0	0
Total liabilities measured at fair value	10,960	10,855
Recurring | NAV Expedient
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, NAV Expedient	37,367	37,326
Company-sponsored investment funds | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	305,109	1,277,219
Total liabilities measured at fair value	3,517	692,130
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	73,114	1,059,616
Total liabilities measured at fair value	205	669,691
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	226,124	215,329
Total liabilities measured at fair value	3,312	22,439
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	5,871	2,264
Total liabilities measured at fair value	0	0
Company-sponsored investment funds | Recurring | NAV Expedient
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, NAV Expedient	0	10
VIEs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	2,628	30,636
Derivatives	2,986	22,241
VIEs | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	196,391	1,189,835
Derivatives	2,628	30,636
Derivatives	2,986	22,241
VIEs | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	19,044	1,053,824
Derivatives	289	252
Derivatives	111	421
VIEs | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	171,609	133,796
Derivatives	2,339	30,384
Derivatives	2,875	21,820
VIEs | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	5,738	2,205
Derivatives	0	0
Derivatives	0	0
VIEs | Recurring | NAV Expedient
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, NAV Expedient	0	10
VIEs | Short | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short equities		669,258
VIEs | Short | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short equities		669,258
VIEs | Short | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short equities		0
VIEs | Short | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short equities		0
VOEs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	856	300
Derivatives	531	631
VOEs | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	105,234	56,448
Derivatives	856	300
Derivatives	531	631
VOEs | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	53,721	5,491
Derivatives	60	49
Derivatives	94	12
VOEs | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	51,380	50,898
Derivatives	796	251
Derivatives	437	619
VOEs | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	133	59
Derivatives	0	0
Derivatives	$ 0	$ 0