CONSOLIDATED STATEMENTS OF SHAREHOLDERS’ EQUITY - USD ($) $ in Millions	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Income (Loss)
Balance (in shares) at Dec. 31, 2020		41,133,668			21,848,314
Balance at Dec. 31, 2020	$ 905.5	$ 0.4	$ 595.5	$ 1,348.9	$ (1,033.7)	$ (5.6)
Comprehensive Income (Loss):
Net income	532.4			532.4
Unrealized gains on changes in fair value of debt securities, net of $0 tax expense	0.2					0.2
Change in fair value of cash flow swaps, net of reclassification adjustment	4.7					4.7
Comprehensive income	537.3			532.4		4.9
Share-based compensation	16.2		16.2
Proceeds from secondary offering of common stock, net (in shares)		3,795,000
Proceeds from secondary offering of common stock, net	666.9		666.9
Issuance of common stock in connection with share-based payment arrangements (in shares)		123,625
Repurchase of common stock associated with net share settlements of employee share-based awards (in shares)					65,937
Repurchase of common stock associated with net share settlements of employee share-based awards	$ (10.4)				$ (10.4)
Purchase of treasury shares (in shares)	0
Balance (in shares) at Dec. 31, 2021		45,052,293			21,914,251
Balance at Dec. 31, 2021	$ 2,115.5	$ 0.4	1,278.6	1,881.3	$ (1,044.1)	(0.7)
Comprehensive Income (Loss):
Net income	997.3			997.3
Unrealized gains on changes in fair value of debt securities, net of $0 tax expense	(4.0)
Change in fair value of cash flow swaps, net of reclassification adjustment	78.1					78.1
Unrealized loss on changes in fair value of debt securities, net of $0.8 million tax benefit	(3.2)					(3.2)
Comprehensive income	1,072.2			997.3		74.9
Share-based compensation	20.6		20.6
Issuance of common stock in connection with share-based payment arrangements (in shares)		122,342
Repurchase of common stock associated with net share settlements of employee share-based awards (in shares)					56,024
Repurchase of common stock associated with net share settlements of employee share-based awards	$ (9.2)				$ (9.2)
Purchase of treasury shares (in shares)	1,635,030				1,635,030
Share issues (repurchases)	$ (295.6)		1.4		$ (297.0)
Retirement of previously repurchased common stock (in shares)		(1,580,826)			(1,580,826)
Retirement of common stock			(19.1)	(268.3)	$ 287.4
Balance (in shares) at Dec. 31, 2022		43,593,809			22,024,479
Balance at Dec. 31, 2022	2,903.5	$ 0.4	1,281.4	2,610.1	$ (1,063.0)	74.4
Comprehensive Income (Loss):
Net income	602.5			602.5
Unrealized gains on changes in fair value of debt securities, net of $0 tax expense	5.2
Change in fair value of cash flow swaps, net of reclassification adjustment	(17.5)					(17.5)
Unrealized loss on changes in fair value of debt securities, net of $0.8 million tax benefit	4.1					4.1
Comprehensive income	589.1			602.5		(13.4)
Share-based compensation	23.5		23.5
Issuance of common stock in connection with share-based payment arrangements (in shares)		128,563
Repurchase of common stock associated with net share settlements of employee share-based awards (in shares)					48,262
Repurchase of common stock associated with net share settlements of employee share-based awards	$ (11.4)				$ (11.4)
Purchase of treasury shares (in shares)	1,316,167				1,316,167
Share issues (repurchases)	$ (260.6)		0.0		$ (260.6)
Retirement of previously repurchased common stock (in shares)		(1,370,371)			(1,370,371)
Retirement of common stock			(16.5)	(251.1)	$ 267.7
Balance (in shares) at Dec. 31, 2023		42,352,001			22,018,537
Balance at Dec. 31, 2023	$ 3,244.1	$ 0.4	$ 1,288.4	$ 2,961.5	$ (1,067.3)	$ 61.0