Stock-Based Compensation - Summary of the company's stock option activity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Shares
Granted (in shares)	0.7	0.3
Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	5.6	7.0
Granted (in shares)	0.7	0.3	0.4
Exercised (in shares)	(0.5)	(1.7)
Cancelled (in shares)	(0.3)	0.0
Options outstanding at the end of the period (in shares)	5.5	5.6	7.0
Options exercisable (in shares)	4.7
Weighted Average Exercise Price
Options outstanding at the beginning of the period (in dollars per share)	$ 18.23	$ 16.00
Granted (in dollars per share)	21.02	29.08
Exercised (in dollars per share)	9.63	10.92
Options canceled weighted average cost (in dollars per share)	23.70	27.07
Options outstanding at the end of the period (in dollars per share)	19.04	$ 18.23	$ 16.00
Options exercisable (in dollars per share)	$ 18.64
Aggregate Intrinsic Value
Options outstanding (in dollars)	$ 11.0
Options exercisable (in dollars)	$ 11.0