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, 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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	237.7			237.7
Change in fair value of cash flow swaps, net of reclassification adjustment	31.8					31.8
Unrealized loss on changes in fair value of debt securities, net of reclassification adjustment and $0.2 million tax benefit	(2.0)					(2.0)
Comprehensive income	267.5			237.7		29.8
Share-based compensation	7.0		7.0
Issuance of common stock, net of forfeitures in connection with share-based payment arrangements (in shares)		(115,435)
Issuance of common stock, net of forfeitures, in connection with share-based payment arrangements	0.0		0.0
Repurchase of common stock associated with net share settlements of employee share-based awards (in shares)					53,810
Repurchase of common stock associated with net share settlement of employee share-based awards	(8.9)				$ (8.9)
Share issues (repurchases) (in shares)					1,069,203
Share repurchases	(198.6)		1.4		$ (200.0)
Retirement of common stock (in shares)		(1,069,203)			(1,069,203)
Retirement of common stock	0.0		(12.9)	(187.1)	$ 200.0
Balance (in shares) at Mar. 31, 2022		44,098,525			21,968,061
Balance at Mar. 31, 2022	2,182.5	$ 0.4	1,274.1	1,931.9	$ (1,053.0)	29.1
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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	644.1
Comprehensive income	$ 718.7
Retirement of common stock (in shares)	(1,069,203)
Balance (in shares) at Sep. 30, 2022		44,102,585			21,969,528
Balance at Sep. 30, 2022	$ 2,642.9	$ 0.4	1,283.4	2,338.2	$ (1,053.3)	74.1
Balance (in shares) at Mar. 31, 2022		44,098,525			21,968,061
Balance at Mar. 31, 2022	2,182.5	$ 0.4	1,274.1	1,931.9	$ (1,053.0)	29.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	201.4			201.4
Change in fair value of cash flow swaps, net of reclassification adjustment	21.7					21.7
Change in fair value of cash flow swaps, net of reclassification adjustment and $6.9 million tax expense	0.2					0.2
Comprehensive income	223.3			201.4		21.9
Share-based compensation	4.7		4.7
Issuance of common stock, net of forfeitures in connection with share-based payment arrangements (in shares)		(1,485)
Repurchase of common stock associated with net share settlements of employee share-based awards (in shares)					436
Repurchase of common stock associated with net share settlement of employee share-based awards	(0.1)				$ (0.1)
Balance (in shares) at Jun. 30, 2022		44,100,010			21,968,497
Balance at Jun. 30, 2022	2,410.4	$ 0.4	1,278.8	2,133.3	$ (1,053.1)	51.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	205.0			205.0
Change in fair value of cash flow swaps, net of reclassification adjustment	25.3					25.3
Change in fair value of cash flow swaps, net of reclassification adjustment and $6.9 million tax expense	(2.5)					(2.5)
Comprehensive income	227.9			205.0		22.9
Share-based compensation	4.7		4.7
Issuance of common stock, net of forfeitures in connection with share-based payment arrangements (in shares)		(2,575)
Repurchase of common stock associated with net share settlements of employee share-based awards (in shares)					1,031
Repurchase of common stock associated with net share settlement of employee share-based awards	$ (0.2)				$ (0.2)
Retirement of common stock (in shares)	0
Balance (in shares) at Sep. 30, 2022		44,102,585			21,969,528
Balance at Sep. 30, 2022	$ 2,642.9	$ 0.4	1,283.4	2,338.2	$ (1,053.3)	74.1
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	181.4			181.4
Change in fair value of cash flow swaps, net of reclassification adjustment	(14.6)					(14.6)
Unrealized loss on changes in fair value of debt securities, net of reclassification adjustment and $0.2 million tax benefit	2.0					2.0
Comprehensive income	168.7			181.4		(12.6)
Share-based compensation	8.6		8.6
Issuance of common stock, net of forfeitures in connection with share-based payment arrangements (in shares)		(120,575)
Issuance of common stock, net of forfeitures, in connection with share-based payment arrangements
Repurchase of common stock associated with net share settlements of employee share-based awards (in shares)					45,613
Repurchase of common stock associated with net share settlement of employee share-based awards	(10.9)				$ (10.9)
Share issues (repurchases) (in shares)					110,323
Share repurchases	(20.7)				$ (20.7)
Retirement of common stock (in shares)		(164,527)			(164,527)
Retirement of common stock	0.0		(2.0)	(28.2)	$ 30.2
Balance (in shares) at Mar. 31, 2023		43,549,857			22,015,888
Balance at Mar. 31, 2023	3,049.2	$ 0.4	1,288.0	2,763.3	$ (1,064.3)	61.8
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	547.0
Comprehensive income	$ 549.8
Retirement of common stock (in shares)	(1,070,126)
Balance (in shares) at Sep. 30, 2023		42,594,062			22,017,396
Balance at Sep. 30, 2023	$ 3,248.5	$ 0.4	1,287.2	2,950.4	$ (1,066.6)	77.2
Balance (in shares) at Mar. 31, 2023		43,549,857			22,015,888
Balance at Mar. 31, 2023	3,049.2	$ 0.4	1,288.0	2,763.3	$ (1,064.3)	61.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	196.4			196.4
Change in fair value of cash flow swaps, net of reclassification adjustment	12.8					12.8
Unrealized loss on changes in fair value of debt securities, net of reclassification adjustment and $0.2 million tax benefit	(3.2)					(3.2)
Comprehensive income	206.0			196.4		9.6
Share-based compensation	5.5		5.5
Issuance of common stock, net of forfeitures in connection with share-based payment arrangements (in shares)		(1,043)
Issuance of common stock, net of forfeitures, in connection with share-based payment arrangements	0.0
Repurchase of common stock associated with net share settlements of employee share-based awards (in shares)					379
Repurchase of common stock associated with net share settlement of employee share-based awards	(0.1)				$ (0.1)
Share issues (repurchases) (in shares)					959,803
Share repurchases	(192.1)				$ (192.1)
Retirement of common stock (in shares)		(959,803)			(959,803)
Retirement of common stock	0.0		(11.6)	(178.5)	$ 190.1
Balance (in shares) at Jun. 30, 2023		42,591,097			22,016,267
Balance at Jun. 30, 2023	3,068.6	$ 0.4	1,282.0	2,781.1	$ (1,066.4)	71.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	169.2			169.2
Change in fair value of cash flow swaps, net of reclassification adjustment	8.6					8.6
Unrealized loss on changes in fair value of debt securities, net of reclassification adjustment and $0.2 million tax benefit	(2.9)					(2.9)
Comprehensive income	175.0			169.2	$ 0.0	5.7
Share-based compensation	5.2		5.2
Issuance of common stock, net of forfeitures in connection with share-based payment arrangements (in shares)		(2,965)
Issuance of common stock, net of forfeitures, in connection with share-based payment arrangements	0.0
Repurchase of common stock associated with net share settlements of employee share-based awards (in shares)					1,129
Repurchase of common stock associated with net share settlement of employee share-based awards	$ (0.2)				$ (0.2)
Retirement of common stock (in shares)	0
Balance (in shares) at Sep. 30, 2023		42,594,062			22,017,396
Balance at Sep. 30, 2023	$ 3,248.5	$ 0.4	$ 1,287.2	$ 2,950.4	$ (1,066.6)	$ 77.2