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, 2014		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
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
Balance at at Dec. 31, 2015	928.1	$ 0.2	100.8	827.2	(0.1)
Net income	332.8	0.0	0.0	332.8	0.0
Other comprehensive income (loss)	(0.1)	0.0	0.0	0.0	(0.1)
Stock-based compensation	$ 7.4	$ 0.0	7.4	0.0	0.0
Restricted stock awards, net of forfeitures, shares		6,643
Repurchase of common stock, shares	(666,330)	(666,330)
Repurchase of common stock	$ (121.7)	$ 0.0	(3.7)	(118.0)	0.0
Restricted stock units converted to common stock, shares		404,096
Tax benefits from stock-based compensation plans	$ 27.2	$ 0.0	27.2	0.0	0.0
Balance, shares at at Dec. 31, 2016	19,877,381	19,877,381
Balance at at Dec. 31, 2016	$ 1,173.7	$ 0.2	131.7	1,042.0	(0.2)
Net income	470.2	0.0	0.0	470.2	0.0
Stock-based compensation	$ 15.4	$ 0.0	15.4	0.0	0.0
Restricted stock awards, net of forfeitures, shares		8,092
Repurchase of common stock, shares	(610,260)	(610,260)
Repurchase of common stock	$ (123.5)	$ 0.0	(1.6)	(121.9)	0.0
Restricted stock units converted to common stock, shares		34,836
Balance, shares at at Dec. 31, 2017	19,310,049	19,310,049
Balance at at Dec. 31, 2017	$ 1,535.8	$ 0.2	$ 145.5	$ 1,390.3	$ (0.2)