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, 2020		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
Balance at Dec. 31, 2021	1,824.2	$ 0.1	197.2	1,626.7	0.2
Net income	535.8	0.0	0.0	535.8	0.0
Other comprehensive income (loss)	(3.1)	0.0	0.0	0.0	(3.1)
Stock-based compensation	$ 36.5	$ 0.0	36.5	0.0	0.0
Repurchase of common stock, shares	(1,491,481)	(1,491,481)
Repurchase of common stock	$ (784.5)	$ 0.0	(3.1)	(781.4)	0.0
Restricted stock units converted to common stock, shares		57,928
Exercised		44,550
Stock Issued During Period, Value, Stock Options Exercised	$ 15.1	$ 0.0	15.1	0.0	0.0
Balance, shares at Dec. 31, 2022	12,756,885	12,756,885
Balance at Dec. 31, 2022	$ 1,624.0	$ 0.1	245.7	1,381.1	(2.9)
Net income	286.1	0.0	0.0	286.1	0.0
Other comprehensive income (loss)	1.9	0.0	0.0	0.0	1.9
Stock-based compensation	$ 39.1	$ 0.0	39.1	0.0	0.0
Repurchase of common stock, shares	(409,317)	(409,317)
Repurchase of common stock	$ (202.6)	$ 0.0	(11.0)	(191.6)	0.0
Restricted stock units converted to common stock, shares		159,205
Exercised		15,624
Stock Issued During Period, Value, Stock Options Exercised	$ 5.2	$ 0.0	5.2	0.0	0.0
Balance, shares at Dec. 31, 2023	12,522,397	12,522,397
Balance at Dec. 31, 2023	$ 1,753.7	$ 0.1	$ 279.0	$ 1,475.6	$ (1.0)