Fair Value Measurements - Categorization of Investments and Certain Other Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equities	$ 49,180	$ 44,992
Derivatives	4,172	933
Securities sold, not yet purchased	3,175	2,755
Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equities	49,180	44,992
Derivatives	4,172	933
Total Assets	402,837	408,016
Securities sold, not yet purchased	3,175	2,755
Derivatives	159,370	102,492
Total Liabilities	162,545	105,247
Debt [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	6,881	5,370
Alternative investment funds [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	56,961	57,890
Debt funds [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	29,806	32,077
Equity funds [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	145,341	154,310
Private equity funds [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	110,496	112,444
Level 1 [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equities	48,996	44,802
Total Assets	215,721	223,549
Securities sold, not yet purchased	3,175	2,696
Total Liabilities	3,175	2,696
Level 1 [Member] | Debt [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,487	1,443
Level 1 [Member] | Debt funds [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	29,802	32,073
Level 1 [Member] | Equity funds [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	135,436	145,231
Level 2 [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	4,172	933
Total Assets	75,132	68,366
Securities sold, not yet purchased		59
Derivatives	159,370	102,492
Total Liabilities	159,370	102,551
Level 2 [Member] | Debt [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	5,394	3,927
Level 2 [Member] | Alternative investment funds [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	55,657	54,433
Level 2 [Member] | Debt funds [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	4	4
Level 2 [Member] | Equity funds [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	9,905	9,069
Level 3 [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equities	184	190
Total Assets	111,984	116,101
Level 3 [Member] | Alternative investment funds [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,304	3,457
Level 3 [Member] | Equity funds [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		10
Level 3 [Member] | Private equity funds [Member] | Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	$ 110,496	$ 112,444