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Financial Instruments - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	$ 1,082,469	$ 759,742
Securities Sold	243,065	173,450
U.S. Government, Agency & Sovereign Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	805,586	525,255
Securities Sold	169,795	131,930
Corporate Debt and Other Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	29,622	14,428
Securities Sold	12,115	1,858
Mortgage and Other Asset-Backed Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	3,626	2,920
Securities Sold	4	18
Municipal Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	77,421	59,010
Securities Sold	629	467
Convertible Bonds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	46,870	49,130
Securities Sold	19,618	8,868
Corporate Equities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	50,740	43,708
Securities Sold	40,663	29,884
Other [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	68,604	65,291
Securities Sold	$ 241	$ 425