Fair Value Measurements - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Details) - USD ($)	Jun. 30, 2016	Dec. 31, 2015
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	$ 952,468,000	$ 735,393,000
Securities Sold	259,032,000	126,493,000
OMHHF
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	562,000	562,000
US Treasury and Government [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	616,831,000	509,614,000
Securities Sold	179,562,000	77,485,000
Corporate Debt and Other Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	40,311,000	16,138,000
Securities Sold	23,811,000	1,652,000
Mortgage and Other Asset-Backed Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	3,875,000	3,504,000
Securities Sold	27,000	27,000
Municipal Bonds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	106,661,000	30,132,000
Securities Sold	0	0
Convertible Bonds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	56,582,000	54,693,000
Securities Sold	12,505,000	5,951,000
Equity Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	38,612,000	34,475,000
Securities Sold	43,127,000	41,378,000
Money Markets [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	495,000	35,000
Securities Sold	0	0
Auction Rate Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	89,101,000	86,802,000
Securities Sold	$ 0	$ 0