Business Segments - Revenue, Pre-tax Earnings and Total Assets by Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Revenues	$ 1,599,110	$ 1,316,232	$ 1,027,336
Pre-tax earnings	379,962	85,187	24,978
Total assets	6,051,178	6,830,654
U.S.
Segment Reporting Information [Line Items]
Revenues	1,449,370	1,127,088	834,410
International
Segment Reporting Information [Line Items]
Revenues	149,740	189,144	192,926
Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,599,110	1,316,232	1,027,336
Pre-tax earnings	379,962	85,187	24,978
Total assets	6,051,178	6,830,654	6,997,004
Operating Segments | Market Making
Segment Reporting Information [Line Items]
Revenues	884,858	901,152	688,197
Pre-tax earnings	124,028	146,713	103,612
Total assets	4,855,482	4,401,021	3,939,965
Operating Segments | Global Execution Services
Segment Reporting Information [Line Items]
Revenues	667,723	345,710	197,765
Pre-tax earnings	368,957	11,056	(25,794)
Total assets	727,029	786,734	1,106,448
Operating Segments | Corporate and Other
Segment Reporting Information [Line Items]
Revenues	46,529	69,369	141,374
Pre-tax earnings	(113,023)	(72,582)	(52,842)
Total assets	$ 468,667	$ 1,642,899	$ 1,950,591