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, 2018		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
Stock-based compensation		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
Balance at Dec. 31, 2019	2,355.3	$ 0.2	157.7	2,196.6	0.8
Net income	421.0	0.0	0.0	421.0	0.0
Other comprehensive income (loss)	0.8	0.0	0.0	0.0	0.8
Stock-based compensation	$ 6.2	$ 0.0	6.2	0.0	0.0
Stock-based compensation		(152)
Repurchase of common stock, shares	(1,282,166)	(1,282,166)
Repurchase of common stock	$ (480.8)	$ 0.0	(2.0)	(478.8)	0.0
Restricted stock units converted to common stock, shares		21,971
Balance, shares at Dec. 31, 2020	17,092,432	17,092,432
Balance at Dec. 31, 2020	$ 2,302.5	$ 0.2	161.9	2,138.8	1.6
Net income	958.3	0.0	0.0	958.3	0.0
Other comprehensive income (loss)	(1.4)	0.0	0.0	0.0	(1.4)
Stock-based compensation	$ 24.8	$ 0.0	24.8	0.0	0.0
Stock-based compensation		(109,085)
Repurchase of common stock, shares	(2,884,126)	(2,884,126)
Repurchase of common stock	$ (1,471.8)	$ (0.1)	(1.3)	(1,470.4)	0.0
Restricted stock units converted to common stock, shares		11,416
Exercised		35,251
Stock Issued During Period, Value, Stock Options Exercised	$ 11.8	$ 0.0	11.8	0.0	0.0
Balance, shares at Dec. 31, 2021	14,145,888	14,145,888
Balance at Dec. 31, 2021	$ 1,824.2	$ 0.1	$ 197.2	$ 1,626.7	$ 0.2