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 Dec. 31, 2016		19,877,381
Balance 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 Dec. 31, 2017		19,310,049
Balance at Dec. 31, 2017	1,535.8	$ 0.2	145.5	1,390.3	(0.2)
Net income	574.0	0.0	0.0	574.0	0.0
Other comprehensive income (loss)	(0.1)	0.0	0.0	0.0	(0.1)
Stock-based compensation	$ 10.3	$ 0.0	10.3	0.0	0.0
Restricted stock awards, net of forfeitures, shares		3,998
Repurchase of common stock, shares	(342,928)	(342,928)
Repurchase of common stock	$ (129.1)	$ 0.0	(0.9)	(128.2)	0.0
Restricted stock units converted to common stock, shares		1,439
Balance, shares at Dec. 31, 2018	18,972,558	18,972,558
Balance at Dec. 31, 2018	$ 1,990.9	$ 0.2	154.9	1,836.1	(0.3)
Net income	656.1	0.0	0.0	656.1	0.0
Other comprehensive income (loss)	1.1	0.0	0.0	0.0	1.1
Stock-based compensation	$ 7.6	$ 0.0	7.6	0.0	0.0
Restricted stock awards, net of forfeitures, shares		4,827
Repurchase of common stock, shares	(712,448)	(712,448)
Repurchase of common stock	$ (300.4)	$ 0.0	(4.8)	(295.6)	0.0
Restricted stock units converted to common stock, shares		87,842
Balance, shares at Dec. 31, 2019	18,352,779	18,352,779
Balance at Dec. 31, 2019	$ 2,355.3	$ 0.2	$ 157.7	$ 2,196.6	$ 0.8