Stock-Based Compensation Plans - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based compensation plans
Shares authorized for future grant	1,475,547
Additional share-based compensation disclosures
Proceed from option exercises	$ 2.8	$ 5.2	$ 37.5
Stock options
Number of shares
Outstanding at the beginning of the year (in shares)	75,275	180,680	934,325
Exercised (in shares)	(48,275)	(98,405)	(753,645)
Expired or forfeited (in shares)	(1,000)	(7,000)
Outstanding at the end of the year (in shares)	26,000	75,275	180,680
Exercisable at the end of the year (in shares)	26,000
Weighted Average Exercise Price
Outstanding at the beginning of the year, Weighted Average Exercise Price (in dollars per share)	$ 52.30	$ 52.61	$ 50.26
Exercised, Weighted Average Exercise Price (in dollars per share)	57.91	52.41	49.70
Expired or forfeited, Weighted Average Exercise Price (in dollars per share)	55.73	58.68
Outstanding at the end of the year, Weighted Average Exercise Price (in dollars per share)	41.76	$ 52.30	$ 52.61
Exercisable at the end of the year, Weighted Average Exercise Price (in dollars per share)	$ 41.76
Weighted Average Remaining Contractual Term
Weighted Average Remaining Contractual Term, stock options outstanding (in years)	10 months 24 days
Weighted Average Remaining Contractual Term, stock options exercisable (in years)	10 months 24 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value, options outstanding	$ 0.8
Aggregate Intrinsic Value, options exercisable	$ 0.8
Changes in non-vested stock options
Non-vested	0	0
Additional share-based compensation disclosures
Proceed from option exercises	$ 2.8	$ 5.2	$ 37.5
Total intrinsic value of all options exercised	1.6	2.8	16.3
Tax benefit realized from option and RSU exercises	$ 4.9	$ 8.4	$ 14.3
Stock options | Non-employee director
Additional share-based compensation disclosures
Vesting period (in years)	1 year
Options with ten-year term | Non-employee director
Additional share-based compensation disclosures
Award expiration term (in years)	10 years