Fair Value of Financial Instruments - Liabilities, Additional Information (Details) (Fair Value, Measurements, Recurring, Unadjusted Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1), USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fair Value, Measurements, Recurring | Unadjusted Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Liabilities measured on a recurring basis
Term Loans	$ 4,633	$ 4,593
Senior Notes	7,022	6,481
Convertible Notes	0	0
Fair value of long-term debt	$ 11,655	$ 11,074