Fair Value Measurements - Schedule of Classification of Investments and Certain Other Assets and Liabilities Measured at Fair Value on Recurring Basis and Investments Measured at NAV (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	$ 354,403	$ 426,630
Total Derivative Assets	11,607	3,732
Total investments measured at fair value	366,010	430,362
Securities sold, not yet purchased	4,274	7,338
Contingent consideration liability	28,974	28,941
Total Derivative Liabilities	214,975	198,417
Total of Liabilities Measured at Fair Value	248,223	234,696
Fair Value, Measurements, Recurring [Member] | NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total investments measured at fair value	76,637	80,378
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total investments measured at fair value	276,167	344,660
Securities sold, not yet purchased	4,274	7,338
Total of Liabilities Measured at Fair Value	4,274	7,338
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Derivative Assets	11,607	3,732
Total investments measured at fair value	11,607	3,732
Total Derivative Liabilities	214,975	198,417
Total of Liabilities Measured at Fair Value	214,975	198,417
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total investments measured at fair value	1,599	1,592
Contingent consideration liability	28,974	28,941
Total of Liabilities Measured at Fair Value	28,974	28,941
Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	7	6
Debt [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	7	6
Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	40,778	45,257
Equities [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	39,179	43,665
Equities [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	1,599	1,592
Alternative Investment Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	17,547	20,993
Alternative Investment Funds [Member] | Fair Value, Measurements, Recurring [Member] | NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	1,007	3,640
Alternative Investment Funds [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	16,540	17,353
Debt Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	62,808	84,077
Debt Funds [Member] | Fair Value, Measurements, Recurring [Member] | NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	7	6
Debt Funds [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	62,801	84,071
Equity Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	157,693	199,618
Equity Funds [Member] | Fair Value, Measurements, Recurring [Member] | NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	53	53
Equity Funds [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	157,640	199,565
Private Equity Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	75,570	76,679
Private Equity Funds [Member] | Fair Value, Measurements, Recurring [Member] | NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	$ 75,570	$ 76,679