CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid in Capital [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Preferred Stock [Member]
Beginning balance, shares at Dec. 31, 2020		100,800,000
Beginning balance at Dec. 31, 2020	$ 3,413.5	$ 0.0	$ 1,390.9	$ 71.1	$ 92.3	$ 2,001.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	653.2					653.2
Restricted stock grants, net, shares		(600,000)
Restricted stock grants, net	25.9		25.9
Treasury Stock, Shares, Acquired		200,000
Treasury Stock, Value, Acquired, Cost Method	14.8			14.8
Stock Issued During Period, Value, New Issues	$ 279.0		279.0
Stock Issued During Period, Shares, New Issues	700,000	3,000,000.0
Dividends, Common Stock, Cash	$ (87.6)					(87.6)
Other comprehensive (loss) income, net	(49.7)				(49.7)
Ending balance, shares at Sep. 30, 2021		104,200,000					12,000,000.0
Ending Balance at Sep. 30, 2021	4,514.0	$ 0.0	1,695.8	85.9	42.6	2,567.0	$ 294.5
Beginning balance, shares at Jun. 30, 2021		104,200,000
Beginning balance at Jun. 30, 2021	4,034.5	$ 0.0	1,687.6	84.2	64.5	2,366.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	236.9					236.9
Restricted stock grants, net, shares		0
Restricted stock grants, net	8.2		8.2
Treasury Stock, Shares, Acquired		0
Treasury Stock, Value, Acquired, Cost Method	1.7			1.7
Dividends, Common Stock, Cash	(36.5)					(36.5)
Other comprehensive (loss) income, net	(21.9)				(21.9)
Ending balance, shares at Sep. 30, 2021		104,200,000					12,000,000.0
Ending Balance at Sep. 30, 2021	4,514.0	$ 0.0	1,695.8	85.9	42.6	2,567.0	$ 294.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Retained earnings	2,773.0
Beginning balance, shares at Dec. 31, 2021		106,600,000					12,000,000.0
Beginning balance at Dec. 31, 2021	4,962.6	$ 0.0	1,966.2	86.8	15.7	2,773.0	$ 294.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	764.3					764.3
Restricted stock grants, net, shares		(600,000)
Restricted stock grants, net	30.3		30.3
Treasury Stock, Shares, Acquired		200,000
Treasury Stock, Value, Acquired, Cost Method	18.4			18.4
Stock Issued During Period, Value, New Issues	$ 157.7		157.7
Stock Issued During Period, Shares, New Issues	1,900,000	1,900,000
Dividends, Common Stock, Cash	$ (114.2)					(114.2)
Other comprehensive (loss) income, net	(751.9)				(751.9)
Ending balance, shares at Sep. 30, 2022		108,900,000					12,000,000.0
Ending Balance at Sep. 30, 2022	5,020.8	$ 0.0	2,154.2	105.2	(736.2)	3,413.5	$ 294.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends, Preferred Stock, Cash	(9.6)					(9.6)
Beginning balance, shares at Jun. 30, 2022		108,300,000					12,000,000.0
Beginning balance at Jun. 30, 2022	4,958.8	$ 0.0	2,094.8	104.3	(517.9)	3,191.7	$ 294.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	264.0					264.0
Restricted stock grants, net, shares		0
Restricted stock grants, net	9.4		9.4
Treasury Stock, Shares, Acquired		0
Treasury Stock, Value, Acquired, Cost Method	0.9			0.9
Stock Issued During Period, Value, New Issues	$ 50.0		50.0
Stock Issued During Period, Shares, New Issues	600,000	600,000
Dividends, Common Stock, Cash	$ (39.0)					(39.0)
Other comprehensive (loss) income, net	(218.3)				(218.3)
Ending balance, shares at Sep. 30, 2022		108,900,000					12,000,000.0
Ending Balance at Sep. 30, 2022	5,020.8	$ 0.0	$ 2,154.2	$ 105.2	$ (736.2)	3,413.5	$ 294.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends, Preferred Stock, Cash	(3.2)					$ (3.2)
Retained earnings	$ 3,413.0