Stock-Based Compensation Stock-Based Compensation - Stock Option Grants and Exercises (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of options granted		1,500,770		550,812		8,173,944
Weighted Average Grant-Date Fair Value per share		$ 10.27	[1]	$ 5.95	[1]	$ 3.50	[1]
Weighted Average Exercise Price per Share		$ 21.18	[1],[2]	$ 13.65	[1],[2]	$ 8.44	[1],[2]
Shares Exercised		317,703		143,109		0
Cash received for options exercised		$ 3		$ 1		$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value		$ 2		$ 2		$ 0
Stock split ratio	1.742

[1] Represents the weighted-average grant date fair value per option, using the Monte Carlo simulation option-pricing model for performance-based options, and the Black-Scholes option-pricing model for time-based options.

[2] Adjusted for the February 2012 1.742-for-1 stock split.