Earnings per Share - Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Diluted earnings (loss) per share:
Net income (loss) available for common stockholders, net of dividends and undistributed earnings allocated to participating securities	$ 631,814	$ (60,521)	$ 284,023
Weighted average shares of common stock outstanding:
Issued and outstanding (in shares)	121,692,443	113,918,103	100,875,793
Weighted average shares of common stock outstanding (in shares)	122,332,190	113,918,103	102,089,139
Diluted earnings (loss) per share (in dollars per share)	$ 5.16	$ (0.53)	$ 2.78
Stock options
Weighted average shares of common stock outstanding:
Dilutive impact excluded from computation of earnings per share (in shares)		377,677
RSUs
Weighted average shares of common stock outstanding:
Dilutive impact excluded from computation of earnings per share (in shares)		440,335
Class A common stock
Weighted average shares of common stock outstanding:
Issued and outstanding (in shares)	121,692,443	113,918,103	100,875,793
Issuable pursuant to Amended and Restated 2015 Management Incentive Plan, Amended and Restated Investment Technology Group, Inc. 2007 Omnibus Equity Compensation Plan, and Warrants issued in connection with the Founder Member Loan (in shares)	639,747	0	1,213,346
Weighted average shares of common stock outstanding (in shares)	122,332,190	113,918,103	102,089,139