Consolidated Company-Sponsored Investment Funds - Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Investments, NAV Expedient	$ 37,200
Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Investments, NAV Expedient	37,326	$ 40,413
Derivatives	11,619	8,413
Short equities	16,376	40,252
Derivatives	14,323	8,649
Total liabilities measured at fair value	55,139	67,182
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Investments	287,162	231,752
Derivatives	948	1,224
Total assets measured at fair value	350,181	340,226
Short equities	16,376	40,252
Derivatives	2,540	1,092
Total liabilities measured at fair value	32,501	42,036
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Investments	63,801	45,768
Derivatives	10,671	7,189
Total assets measured at fair value	890,822	946,146
Short equities	0	0
Derivatives	11,783	7,557
Total liabilities measured at fair value	11,783	7,557
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Investments	1,071	5,023
Derivatives	0	0
Total assets measured at fair value	1,071	5,023
Short equities	0	0
Derivatives	0	0
Total liabilities measured at fair value	10,855	17,589
Company-Sponsored Investment Funds [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Investments, NAV Expedient	10	1,059
Total assets measured at fair value	1,277,219	576,583
Total liabilities measured at fair value	692,130	251,210
Company-Sponsored Investment Funds [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Total assets measured at fair value	1,059,616	352,095
Total liabilities measured at fair value	669,691	248,486
Company-Sponsored Investment Funds [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Total assets measured at fair value	215,329	217,688
Total liabilities measured at fair value	22,439	2,724
Company-Sponsored Investment Funds [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Total assets measured at fair value	2,264	5,741
Total liabilities measured at fair value	0	0
VIEs [Member]
Variable Interest Entity [Line Items]
Derivatives	30,636	5,707
Derivatives	22,241	2,791
VIEs [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Investments	1,189,835	547,650
Investments, NAV Expedient	10	82
Derivatives	30,636	5,707
Derivatives	22,241	2,791
VIEs [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Investments	1,053,824	341,849
Derivatives	252	58
Derivatives	421	67
VIEs [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Investments	133,796	199,978
Derivatives	30,384	5,649
Derivatives	21,820	2,724
VIEs [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Investments	2,205	5,741
Derivatives	0	0
Derivatives	0	0
VIEs [Member] | Short [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Short equities	669,258	248,419
VIEs [Member] | Short [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Short equities	669,258	248,419
VIEs [Member] | Short [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Short equities	0	0
VIEs [Member] | Short [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Short equities	0	0
VOEs [Member]
Variable Interest Entity [Line Items]
Derivatives	300
Derivatives	631
VOEs [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Investments	56,448	23,226
Investments, NAV Expedient		977
Derivatives	300
Derivatives	631
VOEs [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Investments	5,491	10,188
Derivatives	49
Derivatives	12
VOEs [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Investments	50,898	12,061
Derivatives	251
Derivatives	619
VOEs [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Variable Interest Entity [Line Items]
Investments	59	$ 0
Derivatives	0
Derivatives	$ 0