Tender Offer and Warrants and Stock Repurchases - Additional Information (Details) $ / shares in Units, shares in Thousands		1 Months Ended		3 Months Ended		12 Months Ended
	Nov. 04, 2015 USD ($) $ / shares shares	Nov. 30, 2016 shares	Jun. 02, 2015 USD ($) $ / shares shares	Dec. 31, 2016 shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2016 USD ($) director $ / shares shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($)	Jun. 30, 2016	May 04, 2015 USD ($) $ / shares	Apr. 02, 2015 USD ($)	Mar. 13, 2015	Mar. 10, 2015	Jul. 01, 2013 shares
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount											$ 400,000,000.0
Warrants received (in shares) | shares				6,182	21,605	6,182	21,605
Warrants repurchased (in shares) | shares					2,600	15,350
Warrants repurchased					$ 4,400,000	$ 35,100,000
Stock Repurchased and Retired During Period, Value						278,040,000	$ 330,000,000
Professional fees						19,827,000	27,055,000	$ 25,596,000
Total Purchase Price						$ 91,240,000	$ 63,194,000	$ 111,585,000
Senior Secured Notes | 6.875% Senior Secured Notes
Class of Stock [Line Items]
Interest rate (percent)				6.875%	6.875%	6.875%	6.875%		6.875%			6.875%	6.875%
GETCO
Class of Stock [Line Items]
Warrants received (in shares) | shares														24,300
Common Class A
Class of Stock [Line Items]
Remaining authorized repurchase amount											$ 55,000,000.0
Number of Shares Purchased | shares						5,500	3,500
Total Purchase Price						$ 70,700,000	$ 41,700,000
Modified Dutch Auction Tender Offer
Class of Stock [Line Items]
Tender Offer, Odd Lot, Percentage			29.10%
Professional fees							2,100,000
Modified Dutch Auction Tender Offer | Common Class A
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount			$ 330,000,000.0							$ 330,000,000.0
Average price per share (in dollars per share) | $ / shares			$ 14.00
Minimum | Modified Dutch Auction Tender Offer | Common Class A
Class of Stock [Line Items]
Average price per share (in dollars per share) | $ / shares										$ 13.50
Stock Repurchase Program, Number of Shares Authorized to be Repurchased | shares			23,600
Maximum | Modified Dutch Auction Tender Offer | Common Class A
Class of Stock [Line Items]
Average price per share (in dollars per share) | $ / shares										$ 14.00
Common Stock | Common Class A
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Value						187,000	236,000
Common Stock | Modified Dutch Auction Tender Offer | Common Class A
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Value							200,000
Retained Earnings
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Value						$ 255,753,000	329,764,000
Retained Earnings | Modified Dutch Auction Tender Offer
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Value							$ 329,800,000
Stockholder/Former Director | Common Class A
Class of Stock [Line Items]
Number of Shares Purchased | shares						1,900
Total Purchase Price						$ 26,100,000
Treasury Stock Acquired, Average Cost Per Share | $ / shares						$ 13.48
Number of directors | director						2
Serenity Investments, LLC
Class of Stock [Line Items]
Total Purchase Price	$ 24,500,000
Serenity Investments, LLC | Common Class A
Class of Stock [Line Items]
Average price per share (in dollars per share) | $ / shares	$ 12.89
Number of Shares Purchased | shares	1,900
General Atlantic
Class of Stock [Line Items]
Warrants repurchased (in shares) | shares						7,000
Swap Transaction
Class of Stock [Line Items]
Warrants received (in shares) | shares		8,100
Warrants retained (in shares) | shares		1,100		1,100		1,100
Swap Transaction | General Atlantic | Common Class A
Class of Stock [Line Items]
Number of shares exchanged in transaction | shares		18,700		18,700