Fair Value Measurements - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Details) - USD ($)	Sep. 30, 2016	Dec. 31, 2015
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	$ 1,028,496,000	$ 735,393,000
Securities Sold	295,507,000	126,493,000
OMHHF
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	563,000	562,000
US Treasury and Government [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	724,209,000	509,614,000
Securities Sold	223,037,000	77,485,000
Corporate Debt and Other Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	32,262,000	16,138,000
Securities Sold	8,708,000	1,652,000
Mortgage and Other Asset-Backed Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	4,551,000	3,504,000
Securities Sold	46,000	27,000
Municipal Bonds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	85,892,000	30,132,000
Securities Sold	0	0
Convertible Bonds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	53,374,000	54,693,000
Securities Sold	10,454,000	5,951,000
Equity Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	37,057,000	34,475,000
Securities Sold	53,262,000	41,378,000
Money Markets [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	1,142,000	35,000
Securities Sold	0	0
Auction Rate Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	90,009,000	86,802,000
Securities Sold	$ 0	$ 0