Financial Instruments - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	$ 944,471	$ 856,088
Securities Sold	126,092	76,314
U.S. Government, Agency and Sovereign Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	642,125	596,114
Securities Sold	72,327	11,889
Corporate Debt and Other Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	23,791	14,673
Securities Sold	477	4,847
Mortgage and Other Asset-Backed Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	4,914	3,395
Securities Sold	0	7
Municipal Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	63,947	40,166
Securities Sold	73	72
Convertible Bonds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	59,870	53,719
Securities Sold	7,073	13,922
Corporate Equities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	54,629	61,634
Securities Sold	46,116	45,336
Money Markets [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	2,647	1,263
Securities Sold	26	241
Auction Rate Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	$ 92,548	$ 85,124