CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Millions	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (Loss) Restatement Adjustment	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings Restatement Adjustment	Treasury Stock
Balance at Beginning of Year at Dec. 31, 2017		$ 30.5	$ 2,303.3	$ 127.5		$ 110.0	$ 9,542.2		$ 9,556.7	$ (2,428.6)
Balance at Beginning of Year (Accounting Standards Update 2016-01) at Dec. 31, 2017					$ (17.5)			$ 14.5
Increase (Decrease) in Stockholders' Equity
Common Stock Activity		0.0	18.4
Other Comprehensive Income (Loss)	$ (924.2)			(924.2)
Net Income	523.4						523.4
Dividends to Stockholders (per common share: $1.14; $1.09; $0.98)							(217.0)
Purchases of Treasury Stock										(350.7)
Balance at End of Year at Dec. 31, 2018	8,621.8	30.5	2,321.7	(814.2)		(814.2)	9,863.1		9,859.7	(2,779.3)
Balance at End of Year (Accounting Standards Update 2016-01) at Dec. 31, 2018					0.0
Balance at End of Year (Accounting Standards Update 2016-02) at Dec. 31, 2018								(3.4)
Increase (Decrease) in Stockholders' Equity
Common Stock Activity		0.1	26.4
Other Comprehensive Income (Loss)	851.5			851.5
Net Income	1,100.3						1,100.3
Dividends to Stockholders (per common share: $1.14; $1.09; $0.98)							(231.3)
Purchases of Treasury Stock										(400.4)
Balance at End of Year at Dec. 31, 2019	9,965.0	30.6	2,348.1	37.3		$ 37.3	10,728.7		$ 10,709.8	(3,179.7)
Balance at End of Year (Accounting Standards Update 2016-01) at Dec. 31, 2019					$ 0.0
Balance at End of Year (Accounting Standards Update 2016-13) at Dec. 31, 2019								$ (18.9)
Increase (Decrease) in Stockholders' Equity
Common Stock Activity		0.1	28.1
Other Comprehensive Income (Loss)	336.9			336.9
Net Income	793.0						793.0
Dividends to Stockholders (per common share: $1.14; $1.09; $0.98)							(233.2)
Purchases of Treasury Stock										0.0
Balance at End of Year at Dec. 31, 2020	$ 10,871.0	$ 30.7	$ 2,376.2	$ 374.2			$ 11,269.6			$ (3,179.7)