Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments, including money market funds	$ 22	$ 43
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments, including money market funds	0	0
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Investments, including money market funds	0	0
Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Long-term debt	4,914	5,052
Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Long-term debt	$ 5,364	$ 5,441