Stock-Based Compensation - Summary of Option Activity (Details) - Non-qualified stock options - $ / shares shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Number of Options
Outstanding at the beginning of the period	2.3	2.3	2.8
Options granted	0.2	0.3	0.2
Options exercised	(0.8)	(0.3)	(0.7)
Outstanding at the end of the period	1.7	2.3	2.3	2.8
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 26.08	$ 23.75	$ 19.12
Options granted (in dollars per share)	50.50	41.71	43.96
Options exercised (in dollars per share)	22.34	20.17	12.77
Outstanding at the end of the period (in dollars per share)	$ 31.18	$ 26.08	$ 23.75	$ 19.12
Weighted-Average Remaining Contractual Life
Weighted-Average Remaining Contractual Life	5 years 2 months 8 days	5 years 2 months 12 days	5 years 4 months 20 days	4 years 11 months 12 days