Fair Value Measurements - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Details) - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	$ 986,992,000	$ 710,668,000
Securities Sold	234,778,000	85,050,000
OMHHF
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	560,000	3,600,000
Corporate Debt and Other Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	34,051,000	17,074,000
Securities Sold	19,306,000	2,536,000
Mortgage and Other Asset-Backed Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	4,562,000	5,024,000
Securities Sold	2,134,000	31,000
Municipal Bonds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	141,996,000	56,750,000
Convertible Bonds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	45,034,000	56,480,000
Securities Sold	$ 16,456,000	$ 11,604,000