Stock-Based Compensation Plans - Nonqualified Stock Options, Assumptions and Other Disclosures (Details) - Nonqualified Stock Options - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assumptions used to estimate fair value of stock options granted during period
Weighted-average expected volatility (as a percent)	27.60%	31.70%	52.20%
Weighted-average expected term	7 years	6 years 6 months	6 years 6 months
Weighted-average risk-free interest rate (as a percent)	2.20%	1.50%	1.80%
Weighted-average expected dividend yield (as a percent)	2.87%	4.07%	2.81%
Weighted-average estimated fair value of stock options granted (in dollars per share)	$ 15.31	$ 8.91	$ 20.43
Other nonqualified stock option disclosures
Unrecognized compensation costs	$ 2.8
Weighted-average service period over which unrecognized compensation costs will be recognized	1 year 3 months 18 days
Cash received from stock options exercised	$ 133.9	$ 15.3	$ 52.7
Tax benefits realized for the tax deductions from options exercised	$ 18.1	$ 9.0	$ 6.0