Share-Based Compensation Plans (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Maximum of shares available for issuance	33,000,000
Stock awards other than options, conversion ratio	3.5
Shares available for grant	21,419,944
Stock awards other than options, shares available for grant	6,119,984
Share-based employee compensation expense	$ 70.8	$ 72.5	$ 70.5
Unamortized share-based employee compensation, amortization period	5 years
Unamortized share-based employee compensation	$ 168.0
Excess tax (deficiency) benefit from share-based compensation	$ (3.6)	$ 2.8
Option Activity [Roll Forward]
January 1	866,000	988,050	1,593,422
Exercised	0	(57,300)	(500,122)
Forfeited	(97,250)	(64,750)	(105,250)
December 31	768,750	866,000	988,050
Exercisable December 31	768,750	60,000	117,300
January 1, Weighted Average Exercise Price	$ 83.80	$ 80.37	$ 63.11
Exercised, Weighted Average Exercise Price	0	23.40	24.43
Forfeited, Weighted Average Exercise Price	84.94	84.94	84.94
December 31, Weighted Average Exercise Price	83.65	83.80	80.37
Exercisable December 31, Weighted Average Exercise Price	$ 83.65	$ 68.42	$ 46.43
Options Outstanding and Exercisable [Abstract]
Options Outstanding, Shares	768,750
Options Exercisable, Shares	768,750
Exercise Price Range - $66.00 to $71.00
Options Outstanding and Exercisable [Abstract]
Range of Exercise Prices, lower range limit	$ 66.00
Range of Exercise Prices, upper range limit	$ 71.00
Options Outstanding, Shares	60,000
Options Outstanding, Weighted Average Remaining Contractual Life	3 years 4 months 24 days
Options Outstanding, Weighted Average Exercise Price	$ 68.42
Options Exercisable, Shares	60,000
Options Exercisable, Weighted Average Exercise Price	$ 68.42
Exercise Price Range - $84.00 to $85.00
Options Outstanding and Exercisable [Abstract]
Range of Exercise Prices, lower range limit	84.00
Range of Exercise Prices, upper range limit	$ 85.00
Options Outstanding, Shares	708,750
Options Outstanding, Weighted Average Remaining Contractual Life	2 years 3 months 18 days
Options Outstanding, Weighted Average Exercise Price	$ 84.94
Options Exercisable, Shares	708,750
Options Exercisable, Weighted Average Exercise Price	$ 84.94
Stock Options [Member]
Share-based employee compensation, terms of award	The exercise price of stock option awards cannot be less than 100% of the market price of our common stock on the grant date and have a maximum contractual life of 10 years.
Restricted Stock [Member]
Share-based employee compensation, vesting rights	Generally, restricted shares vest ratably over five years from the grant date provided the employee remains employed by us.
Restricted Stock and PRSU Activity [Roll Forward]
January 1	2,547,001	2,553,902	2,859,373
Granted	1,511,719	956,135	815,810
Vested / Distributed	(874,431)	(798,468)	(944,048)
Forfeited	(171,301)	(164,568)	(177,233)
December 31	3,012,988	2,547,001	2,553,902
Restricted Stock and PRSU Activity, Weighted Average Grant Date Fair Value [Roll Forward]
January 1, Weighted Average Grant Date Fair Value	$ 70.89	$ 69.77
Granted, Weighted Average Grant Date Fair Value	51.26	72.13	$ 67.62
December 31, Weighted Average Grant Date Fair Value	$ 61.44	$ 70.89	$ 69.77
Performance Restricted Stock Units (PRSUs) [Member]
Share-based employee compensation, vesting rights	Each PRSU represents the right to receive one share of common stock on vesting. The ultimate number of PRSUs received by the employee depends on the Company's average return on equity over a three year period compared to the average return on equity of a peer group of principal competitors over the same period. The PRSUs vest three years from the grant date.
Restricted Stock and PRSU Activity [Roll Forward]
January 1	538,134	509,844	488,887
Granted	186,197	181,782	182,582
Vested / Distributed	(173,770)	(153,492)	(161,625)
December 31	550,561	538,134	509,844
Restricted Stock and PRSU Activity, Weighted Average Grant Date Fair Value [Roll Forward]
January 1, Weighted Average Grant Date Fair Value	$ 77.99	$ 80.41	$ 82.01
Granted, Weighted Average Grant Date Fair Value	61.36	75.64	73.72
Distributed, Weighted Average Grant Date Fair Value	84.94	83.23	77.68
December 31, Weighted Average Grant Date Fair Value	$ 70.17	$ 77.99	$ 80.41
Employee Stock Purchase Plan [Member]
Shares available for grant	8,507,706
Employee Stock Purchase Plan (ESPP) [Abstract]
Discount from market price	95.00%
Maximum percentage of eligible compensation allowable for purchase	10.00%
Shares issued in period	91,605	76,040	91,086
Proceeds from issuance of shares	$ 5.2	$ 5.6	$ 6.5