Financial Instruments - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	$ 806,772	$ 856,088
Securities Sold	128,262	76,314
U.S. Government, Agency and Sovereign Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	522,010	596,114
Securities Sold	67,496	11,889
Corporate Debt and Other Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	19,922	14,673
Securities Sold	3,010	4,847
Mortgage and Other Asset-Backed Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	4,792	3,395
Securities Sold	0	7
Municipal Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	62,250	40,166
Securities Sold	55	72
Convertible Bonds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	58,795	53,719
Securities Sold	8,229	13,922
Corporate Equities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	49,012	61,634
Securities Sold	49,457	45,336
Money Markets [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	2,668	1,263
Securities Sold	15	241
Auction Rate Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	$ 87,323	$ 85,124