Consolidated Statements Of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance, shares at at Dec. 31, 2012		24,114,896
Balance at at Dec. 31, 2012	$ 621.9	$ 0.2	$ 53.4	$ 568.4	$ (0.1)
Net income	253.1	0.0	0.0	253.1	0.0
Other comprehensive income (loss)	(0.1)	0.0	0.0	0.0	(0.1)
Stock-based compensation	$ 8.5	$ 0.0	8.5	0.0	0.0
Restricted stock awards, net of forfeitures, shares		8,349
Repurchase of common stock, shares	(1,216,015)	(1,216,015)
Repurchase of common stock	$ (135.2)	$ 0.0	(0.6)	(134.6)	0.0
Restricted stock units converted to common stock, shares		848
Stock options exercised, shares		35,000
Stock options exercised	0.6	$ 0.0	0.6	0.0	0.0
Tax benefits from stock-based compensation plans	1.3	$ 0.0	1.3	0.0	0.0
Balance, shares at at Dec. 31, 2013		22,943,078
Balance at at Dec. 31, 2013	750.1	$ 0.2	63.2	686.9	(0.2)
Net income	266.2	0.0	0.0	266.2	0.0
Other comprehensive income (loss)	0.1	0.0	0.0	0.0	0.1
Stock-based compensation	$ 15.3	$ 0.0	15.3	0.0	0.0
Restricted stock awards, net of forfeitures, shares		5,462
Repurchase of common stock, shares	(2,686,744)	(2,686,744)
Repurchase of common stock	$ (343.7)	$ 0.0	(4.0)	(339.7)	0.0
Restricted stock units converted to common stock, shares		300,875
Stock options exercised, shares		35,000
Stock options exercised	0.6	$ 0.0	0.6	0.0	0.0
Tax benefits from stock-based compensation plans	$ 13.6	$ 0.0	13.6	0.0	0.0
Balance, shares at at Dec. 31, 2014	20,597,671	20,597,671
Balance at at Dec. 31, 2014	$ 702.2	$ 0.2	88.7	613.4	(0.1)
Net income	299.7	0.0	0.0	299.7	0.0
Other comprehensive income (loss)	0.0	0.0	0.0	0.0	0.0
Stock-based compensation	$ 12.4	$ 0.0	12.4	0.0	0.0
Restricted stock awards, net of forfeitures, shares		4,024
Repurchase of common stock, shares	(470,071)	(470,071)
Repurchase of common stock	$ (86.5)	$ 0.0	(0.6)	(85.9)	0.0
Restricted stock units converted to common stock, shares		1,348
Tax benefits from stock-based compensation plans	$ 0.3	$ 0.0	0.3	0.0	0.0
Balance, shares at at Dec. 31, 2015	20,132,972	20,132,972
Balance at at Dec. 31, 2015	$ 928.1	$ 0.2	$ 100.8	$ 827.2	$ (0.1)