Fair Value Measurements (Marketable Securities) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Mar. 31, 2014
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated Fair Value	$ 734	$ 3,309
Available-for-sale Securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated Fair Value	651	3,205
Current Assets, Prepaid Expenses, And Other Current Assets
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated Fair Value	3	24
Other Assets
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated Fair Value	80	80
Available-for-sale Securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated Fair Value	654	3,229
Available-for-sale Securities | Asset-backed Securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Available-for-sale Securities, Amortized Cost Basis	264	1,523
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	3
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	(1)
Estimated Fair Value	264	1,525
Available-for-sale Securities | Foreign Government Debt Securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Available-for-sale Securities, Amortized Cost Basis	101	725
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	2
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	(1)
Estimated Fair Value	101	726
Available-for-sale Securities | US Government Agencies Debt Securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Available-for-sale Securities, Amortized Cost Basis	185	382
Estimated Fair Value	185	382
Available-for-sale Securities | United States Debt
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Available-for-sale Securities, Amortized Cost Basis	72	474
Estimated Fair Value	72	474
Available-for-sale Securities | Certificate of deposit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Available-for-sale Securities, Amortized Cost Basis	28	110
Estimated Fair Value	28	110
Available-for-sale Securities | Other Security Investments
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Available-for-sale Securities, Amortized Cost Basis	4	12
Estimated Fair Value	4		$ 12
Trading Securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated Fair Value	$ 80	$ 80