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	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	$ 609,076	$ 426,630
Total Derivative Assets	7,490	3,732
Total investments measured at fair value	616,566	430,362
Securities sold, not yet purchased	6,876	7,338
Contingent consideration liability	12,921	28,941
Total Derivative Liabilities	206,729	198,417
Total of Liabilities Measured at Fair Value	226,526	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	60,649	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	546,799	344,660
Securities sold, not yet purchased	6,876	7,338
Total of Liabilities Measured at Fair Value	6,876	7,338
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Derivative Assets	7,490	3,732
Total investments measured at fair value	7,490	3,732
Total Derivative Liabilities	206,729	198,417
Total of Liabilities Measured at Fair Value	206,729	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	1,628	1,592
Contingent consideration liability	12,921	28,941
Total of Liabilities Measured at Fair Value	12,921	28,941
Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	202,592	6
Debt [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	202,592	6
Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	36,810	45,257
Equities [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	35,182	43,665
Equities [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	1,628	1,592
Alternative Investment Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	18,169	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	915	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	17,254	17,353
Debt Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	89,476	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	89,469	84,071
Equity Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	202,354	199,618
Equity Funds [Member] | Fair Value, Measurements, Recurring [Member] | NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	52	53
Equity Funds [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	202,302	199,565
Private Equity Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	59,675	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	$ 59,675	$ 76,679