Stock-Based Compensation Plans - Nonqualified Stock Options, Range of Exercise Prices (Details) - Nonqualified Stock Options - $ / shares shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Summary of weighted-average remaining contractual lives for stock options outstanding and range of exercise prices
Number of options outstanding (in shares)	4.3	7.5
Weighted-average remaining contractual life for stock options exercisable	5 years
Range of exercise prices, Range 1
Summary of weighted-average remaining contractual lives for stock options outstanding and range of exercise prices
Lower limit of exercise price range (in dollars per share)	$ 11.07
Upper limit of exercise price range (in dollars per share)	$ 21.69
Number of options outstanding (in shares)	0.1
Weighted-average remaining contractual life	1 year 2 months 12 days
Range of exercise prices, Range 2
Summary of weighted-average remaining contractual lives for stock options outstanding and range of exercise prices
Lower limit of exercise price range (in dollars per share)	$ 21.70
Upper limit of exercise price range (in dollars per share)	$ 32.32
Number of options outstanding (in shares)	1.0
Weighted-average remaining contractual life	4 years 7 months 6 days
Range of exercise prices, Range 3
Summary of weighted-average remaining contractual lives for stock options outstanding and range of exercise prices
Lower limit of exercise price range (in dollars per share)	$ 32.33
Upper limit of exercise price range (in dollars per share)	$ 42.95
Number of options outstanding (in shares)	1.2
Weighted-average remaining contractual life	7 years 6 months
Range of exercise prices, Range 4
Summary of weighted-average remaining contractual lives for stock options outstanding and range of exercise prices
Lower limit of exercise price range (in dollars per share)	$ 42.96
Upper limit of exercise price range (in dollars per share)	$ 53.58
Number of options outstanding (in shares)	1.0
Weighted-average remaining contractual life	6 years 8 months 12 days
Range of exercise prices, Range 5
Summary of weighted-average remaining contractual lives for stock options outstanding and range of exercise prices
Lower limit of exercise price range (in dollars per share)	$ 53.59
Upper limit of exercise price range (in dollars per share)	$ 64.22
Number of options outstanding (in shares)	1.0
Weighted-average remaining contractual life	6 years 8 months 12 days
Range of exercise prices, Total Range
Summary of weighted-average remaining contractual lives for stock options outstanding and range of exercise prices
Lower limit of exercise price range (in dollars per share)	$ 11.07
Upper limit of exercise price range (in dollars per share)	$ 64.22
Number of options outstanding (in shares)	4.3
Weighted-average remaining contractual life	6 years 3 months 18 days