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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	Mar. 31, 2014	Dec. 31, 2013
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	$ 936,016	$ 856,088
Securities Sold	143,453	76,314
U.S. Government, Agency and Sovereign Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	689,584	596,114
Securities Sold	85,502	11,889
Corporate Debt and Other Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	15,151	14,673
Securities Sold	2,325	4,847
Mortgage and Other Asset-Backed Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	5,245	3,395
Securities Sold	4	7
Municipal Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	43,494	40,166
Securities Sold	113	72
Convertible Bonds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	55,965	53,719
Securities Sold	8,052	13,922
Corporate Equities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	40,443	61,634
Securities Sold	47,249	45,336
Money Markets [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	1,109	1,263
Securities Sold	208	241
Auction Rate Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	85,025	85,124
Securities Sold