Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Derivatives	$ 64
Fair Value, Measurements, Recurring [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Marketable Securities	266	266
Derivatives	64	86
Total Assets at Fair Value	330	352
Derivatives	110	149
Total Liabilities at Fair Value	110	149
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Marketable Securities	266	266
Derivatives	2	21
Total Assets at Fair Value	268	287
Derivatives	106	148
Total Liabilities at Fair Value	106	148
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Marketable Securities	0	0
Derivatives	62	65
Total Assets at Fair Value	62	65
Derivatives	4	1
Total Liabilities at Fair Value	4	1
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Assets and Liabilities Measured at Fair Value on a Recurring Basis
Marketable Securities	0	0
Derivatives	0	0
Total Assets at Fair Value	0	0
Derivatives	0	0
Total Liabilities at Fair Value	$ 0	$ 0