CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Millions	Total		Revision of Prior Period, Accounting Standards Update, Adjustment	Previously Reported [Member]	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Other Comprehensive Loss Revision of Prior Period, Accounting Standards Update, Adjustment	Accumulated Other Comprehensive Loss Previously Reported [Member]	Retained Earnings	Retained Earnings Revision of Prior Period, Accounting Standards Update, Adjustment	Retained Earnings Previously Reported [Member]	Retained Earnings Cumulative Effect, Period of Adoption, Adjusted Balance	Treasury Stock, Common
Balance at Beginning of Year at Dec. 31, 2021					$ 30.7	$ 2,408.1	$ (5,164.6)		$ 354.1			$ 11,853.2		$ (3,229.7)
Balance at Beginning of Year (Accounting Standards Update 2018-12) at Dec. 31, 2021								$ (5,518.7)			$ 136.2		$ 11,989.4
Increase (Decrease) in Stockholders' Equity
Common Stock Activity					0.1	(0.2)
Other Comprehensive Income	$ 726.1			$ (934.2)			726.1
Other Comprehensive Income | Accounting Standards Update 2018-12			$ 1,660.3
Net Income	240.4			253.5						$ 240.4
Net Income | Accounting Standards Update 2018-12			(13.1)
Dividends to Stockholders (per common share: $0.330; $0.300)										(62.9)
Balance at End of Year at Mar. 31, 2022	6,887.8			10,623.1	30.8	2,395.8	(4,438.5)			12,166.9				(3,267.2)
Balance at End of Year (Accounting Standards Update 2018-12) at Mar. 31, 2022			(3,735.3)
Increase (Decrease) in Stockholders' Equity
Cost of Shares Repurchased	(37.5)	[1]				12.5								(37.5)
Balance at Beginning of Year at Dec. 31, 2021					30.7	2,408.1	(5,164.6)		354.1			11,853.2		(3,229.7)
Balance at Beginning of Year (Accounting Standards Update 2018-12) at Dec. 31, 2021								(5,518.7)			136.2		11,989.4
Balance at End of Year at Dec. 31, 2022	8,735.0			$ 9,197.5	30.8	2,441.0	(3,448.3)		$ (3,448.3)			$ 13,141.3		(3,429.8)
Balance at End of Year (Accounting Standards Update 2018-12) at Dec. 31, 2022			$ (462.5)					$ 0.0			$ 0.0		$ 13,141.3
Increase (Decrease) in Stockholders' Equity
Common Stock Activity					0.1	9.2
Other Comprehensive Income	286.0						286.0
Net Income	358.3									358.3
Dividends to Stockholders (per common share: $0.330; $0.300)										(69.2)
Balance at End of Year at Mar. 31, 2023	9,247.4				$ 30.9	2,431.8	$ (3,162.3)			$ 13,430.4				(3,483.4)
Increase (Decrease) in Stockholders' Equity
Cost of Shares Repurchased	$ (53.6)	[1]				$ 0.0								$ (53.6)

[1] 1Includes a de minimis amount of commissions and $0.1 million of excise tax for the three months ended March 31, 2023. There were no commissions or excise taxes during the three months ended March 31, 2022.