Stockholders' Equity - Follow-on Offering (Details) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended		12 Months Ended
	Dec. 31, 2013	May 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 23, 2013	May 17, 2012
Class of Stock
Net proceeds from issuance of common stock	$ 1,480	$ 6,760	$ 0	$ 1,478	$ 6,760
Underwriting discounts and commissions	7	75
Other offering costs	$ 1	$ 7
Class A Common Stock
Class of Stock
Issuance of common stock, shares	27,004,761	180,000,000
Share price			$ 78.02			$ 55.05	$ 38.00
Sale of stock, sold by stockholders	42,995,239	241,233,615