CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Millions	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings Cumulative Effect, Period of Adoption, Adjusted Balance	Treasury Stock
Balance at Beginning of Year at Dec. 31, 2019		$ 30.6	$ 2,348.1	$ 37.3	$ 10,728.7			$ (3,179.7)
Balance at Beginning of Year (Accounting Standards Update 2016-13) at Dec. 31, 2019						$ (18.9)	$ 10,709.8
Increase (Decrease) in Stockholders' Equity
Common Stock Activity			16.0
Other Comprehensive Income (Loss)	$ 289.9			289.9
Net Income	426.5				426.5
Dividends to Stockholders (per common share: $0.285; $0.285; $0.570; $0.570)					(117.1)
Balance at End of Year at Jun. 30, 2020	10,561.4		2,364.1	327.2	11,019.2
Balance at Beginning of Year at Mar. 31, 2020		30.6	2,357.3	(217.1)	10,812.0			(3,179.7)
Balance at Beginning of Year (Accounting Standards Update 2016-13) at Mar. 31, 2020						0.0	10,812.0
Increase (Decrease) in Stockholders' Equity
Common Stock Activity			6.8
Other Comprehensive Income (Loss)	544.3			544.3
Net Income	265.5				265.5
Dividends to Stockholders (per common share: $0.285; $0.285; $0.570; $0.570)					(58.3)
Balance at End of Year at Jun. 30, 2020	10,561.4		2,364.1	327.2	11,019.2
Balance at Beginning of Year at Dec. 31, 2020	10,871.0	30.7	2,376.2	374.2	11,269.6			(3,179.7)
Balance at Beginning of Year (Accounting Standards Update 2016-13) at Dec. 31, 2020						0.0	11,269.6
Increase (Decrease) in Stockholders' Equity
Common Stock Activity			10.6
Other Comprehensive Income (Loss)	(152.1)			(152.1)
Net Income	335.9				335.9
Dividends to Stockholders (per common share: $0.285; $0.285; $0.570; $0.570)					(117.9)
Balance at End of Year at Jun. 30, 2021	10,947.5		2,386.8	222.1	11,487.6
Balance at Beginning of Year at Mar. 31, 2021		$ 30.7	2,380.8	(24.9)	11,363.2			$ (3,179.7)
Balance at Beginning of Year (Accounting Standards Update 2016-13) at Mar. 31, 2021						$ 0.0	$ 11,363.2
Increase (Decrease) in Stockholders' Equity
Common Stock Activity			6.0
Other Comprehensive Income (Loss)	247.0			247.0
Net Income	182.9				182.9
Dividends to Stockholders (per common share: $0.285; $0.285; $0.570; $0.570)					(58.5)
Balance at End of Year at Jun. 30, 2021	$ 10,947.5		$ 2,386.8	$ 222.1	$ 11,487.6