Stock-Based Compensation - Summary of Option Activity (Details) (Non-qualified stock options, USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Non-qualified stock options
Summary of option activity
Outstanding at the beginning of the period	3.0	3.3	3.4
Options granted	0.2	0.3	0.5
Options exercised	(0.4)	(0.6)	(0.6)
Outstanding at the end of the period	2.8	3.0	3.3	3.4
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 17.30	$ 15.67	$ 13.55
Options granted (in dollars per share)	$ 43.01	$ 28.27	$ 25.03
Options exercised (in dollars per share)	$ 14.66	$ 10.41	$ 10.41
Options expired or forfeited (in dollars per share)			$ 21.82
Outstanding at the end of the period (in dollars per share)	$ 19.12	$ 17.30	$ 15.67	$ 13.55
Weighted-Average Remaining Contractual Life
Weighted-Average Remaining Contractual Life	4 years 11 months 12 days	5 years 11 months 16 days	6 years 3 months 4 days	6 years 5 months 12 days