Stockholders' Equity - Follow-on Offering (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Class of Stock
Net proceeds from issuance of common stock		$ 0	$ 0	$ 1,478
Class A Common Stock
Class of Stock
Issuance of common stock, shares	27,004,761
Share price	$ 55.05	$ 104.66		$ 55.05
Sale of stock, sold by stockholders	42,995,239
Net proceeds from issuance of common stock	$ 1,478
Underwriting discounts and commissions	7
Other offering costs	$ 1