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	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	$ 426,630	$ 458,744
Total Derivative Assets	3,732	1,993
Total investments measured at fair value	430,362	460,737
Securities sold, not yet purchased	7,338	4,482
Contingent consideration liability	28,941	22,608
Total Derivative Liabilities	198,417	182,223
Total of Liabilities Measured at Fair Value	234,696	209,313
Fair Value, Measurements, Recurring [Member] | NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total investments measured at fair value	80,378	129,591
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	344,660	327,645
Securities sold, not yet purchased	7,338	4,482
Total of Liabilities Measured at Fair Value	7,338	4,482
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Derivative Assets	3,732	1,993
Total investments measured at fair value	3,732	1,993
Total Derivative Liabilities	198,417	182,223
Total of Liabilities Measured at Fair Value	198,417	182,223
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	1,592	1,508
Contingent consideration liability	28,941	22,608
Total of Liabilities Measured at Fair Value	28,941	22,608
Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	6
Debt [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	6
Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	45,257	41,017
Equities [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	43,665	39,509
Equities [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	1,592	1,508
Alternative Investment Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	20,993	32,441
Alternative Investment Funds [Member] | Fair Value, Measurements, Recurring [Member] | NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	3,640	7,125
Alternative Investment Funds [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	17,353	25,316
Debt Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	84,077	74,597
Debt Funds [Member] | Fair Value, Measurements, Recurring [Member] | NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	6	6
Debt Funds [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	84,071	74,591
Equity Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	199,618	188,268
Equity Funds [Member] | Fair Value, Measurements, Recurring [Member] | NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	53	39
Equity Funds [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	199,565	188,229
Private Equity Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	76,679	122,421
Private Equity Funds [Member] | Fair Value, Measurements, Recurring [Member] | NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments	$ 76,679	$ 122,421