Consolidated Company-Sponsored Investment Funds - Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Recurring
Variable Interest Entity [Line Items]
Derivatives	$ 17,079	$ 11,619
Short equities	4,841	16,376
Derivatives	14,822	14,323
Total liabilities measured at fair value	30,781	55,139
Level 1 | Recurring
Variable Interest Entity [Line Items]
Investments	216,251	278,655
Derivatives	1,594	948
Total assets measured at fair value	320,733	341,674
Short equities	4,841	16,376
Derivatives	2,534	2,540
Total liabilities measured at fair value	11,157	32,501
Level 2 | Recurring
Variable Interest Entity [Line Items]
Investments	400,796	72,308
Derivatives	15,485	10,671
Total assets measured at fair value	1,585,835	899,329
Short equities	0	0
Derivatives	12,288	11,783
Total liabilities measured at fair value	12,288	11,783
Level 3 | Recurring
Variable Interest Entity [Line Items]
Investments	142	1,071
Derivatives	0	0
Total assets measured at fair value	142	1,071
Short equities	0	0
Derivatives	0	0
Total liabilities measured at fair value	7,336	10,855
NAV Expedient | Recurring
Variable Interest Entity [Line Items]
Investments, NAV Expedient	315	37,326
Company-Sponsored Investment Funds | Recurring
Variable Interest Entity [Line Items]
Total assets measured at fair value	358,922	1,277,219
Total liabilities measured at fair value	7,721	692,130
Company-Sponsored Investment Funds | Level 1 | Recurring
Variable Interest Entity [Line Items]
Total assets measured at fair value	91,822	1,059,616
Total liabilities measured at fair value	269	669,691
Company-Sponsored Investment Funds | Level 2 | Recurring
Variable Interest Entity [Line Items]
Total assets measured at fair value	258,727	215,329
Total liabilities measured at fair value	7,452	22,439
Company-Sponsored Investment Funds | Level 3 | Recurring
Variable Interest Entity [Line Items]
Total assets measured at fair value	8,373	2,264
Total liabilities measured at fair value	0	0
VIEs
Variable Interest Entity [Line Items]
Derivatives	3,410	30,636
Derivatives	3,891	22,241
VIEs | Recurring
Variable Interest Entity [Line Items]
Investments	217,840	1,189,835
Derivatives	3,410	30,636
Derivatives	3,891	22,241
VIEs | Level 1 | Recurring
Variable Interest Entity [Line Items]
Investments	22,149	1,053,824
Derivatives	1,486	252
Derivatives	72	421
VIEs | Level 2 | Recurring
Variable Interest Entity [Line Items]
Investments	187,626	133,796
Derivatives	1,924	30,384
Derivatives	3,819	21,820
VIEs | Level 3 | Recurring
Variable Interest Entity [Line Items]
Investments	8,065	2,205
Derivatives	0	0
Derivatives	0	0
VIEs | NAV Expedient | Recurring
Variable Interest Entity [Line Items]
Investments, NAV Expedient	0	10
VIEs | Short | Recurring
Variable Interest Entity [Line Items]
Short equities		669,258
VIEs | Short | Level 1 | Recurring
Variable Interest Entity [Line Items]
Short equities		669,258
VIEs | Short | Level 2 | Recurring
Variable Interest Entity [Line Items]
Short equities		0
VIEs | Short | Level 3 | Recurring
Variable Interest Entity [Line Items]
Short equities		0
VOEs
Variable Interest Entity [Line Items]
Derivatives	3,816	300
Derivatives	3,830	631
VOEs | Recurring
Variable Interest Entity [Line Items]
Investments	133,856	56,448
Derivatives	3,816	300
Derivatives	3,830	631
VOEs | Level 1 | Recurring
Variable Interest Entity [Line Items]
Investments	68,063	5,491
Derivatives	124	49
Derivatives	197	12
VOEs | Level 2 | Recurring
Variable Interest Entity [Line Items]
Investments	65,485	50,898
Derivatives	3,692	251
Derivatives	3,633	619
VOEs | Level 3 | Recurring
Variable Interest Entity [Line Items]
Investments	308	59
Derivatives	0	0
Derivatives	$ 0	$ 0