CONSOLIDATED STATEMENTS OF SHAREHOLDERS’ EQUITY - USD ($)	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Additional Paid-in Capital	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Treasury Stock	Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Cumulative Effect, Period of Adoption, Adjustment
Accounting Standards Update [Extensible List]	Accounting Standards Update 2018-02 [Member]
Balance (in shares) at Dec. 31, 2018			41,065,069				21,719,339
Balance at Dec. 31, 2018	$ 473,200,000	$ 0	$ 400,000	$ 572,900,000	$ 922,700,000	$ 200,000	$ (1,023,400,000)	$ 600,000	$ (200,000)
Comprehensive Income (Loss):
Net income	184,400,000				184,400,000
Unrealized gains on available-for-sale debt securities, net of $0 tax charge	0
Change in fair value of cash flow swaps, net of reclassification adjustment	(3,300,000)							(3,300,000)
Comprehensive income	181,100,000				184,400,000			(3,300,000)
Share-based compensation	12,500,000			12,500,000
Issuance of common stock in connection with share-based payment arrangements (in shares)			209,390
Issuance of common stock, net of forfeitures, in connection with share-based payment arrangements	0
Repurchase of common stock associated with net share settlements of employee share-based awards (in shares)							72,368
Repurchase of common stock associated with net share settlements of employee share-based awards	(5,200,000)						$ (5,200,000)
Purchase of treasury shares (in shares)							202,379
Purchase of treasury shares	(15,300,000)						$ (15,300,000)
Retirement of previously repurchased common stock (in shares)			(202,379)				(202,379)
Retirement of previously repurchased common stock	0			(2,500,000)	(12,800,000)		$ 15,300,000
Balance (in shares) at Dec. 31, 2019			41,072,080				21,791,707
Balance at Dec. 31, 2019	646,300,000		$ 400,000	582,900,000	1,094,500,000		$ (1,028,600,000)	(2,900,000)
Comprehensive Income (Loss):
Net income	254,400,000				254,400,000
Unrealized gains on available-for-sale debt securities, net of $0 tax charge	0
Change in fair value of cash flow swaps, net of reclassification adjustment	(2,700,000)							(2,700,000)
Comprehensive income	251,700,000				254,400,000			(2,700,000)
Share-based compensation	12,600,000			12,600,000
Issuance of common stock in connection with share-based payment arrangements (in shares)			61,588
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)							56,607
Repurchase of common stock associated with net share settlements of employee share-based awards	(5,100,000)						$ (5,100,000)
Balance (in shares) at Dec. 31, 2020			41,133,668				21,848,314
Balance at Dec. 31, 2020	905,500,000		$ 400,000	595,500,000	1,348,900,000		$ (1,033,700,000)	(5,600,000)
Comprehensive Income (Loss):
Net income	532,400,000				532,400,000
Unrealized gains on available-for-sale debt securities, net of $0 tax charge	200,000							200,000
Change in fair value of cash flow swaps, net of reclassification adjustment	4,700,000							4,700,000
Comprehensive income	537,300,000				532,400,000			4,900,000
Share-based compensation	16,200,000			16,200,000
Proceeds from secondary offering of common stock, net (in shares)			3,795,000
Proceeds from secondary offering of common stock, net	666,900,000			666,900,000
Issuance of common stock in connection with share-based payment arrangements (in shares)			123,625
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)							65,937
Repurchase of common stock associated with net share settlements of employee share-based awards	(10,400,000)						$ (10,400,000)
Retirement of previously repurchased common stock	0
Balance (in shares) at Dec. 31, 2021			45,052,293				21,914,251
Balance at Dec. 31, 2021	$ 2,115,500,000		$ 400,000	$ 1,278,600,000	$ 1,881,300,000		$ (1,044,100,000)	$ (700,000)