Fair Value Measurement (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Assets:
Commodity derivatives	$ 6
Total	6
Liabilities:
Commodity derivatives	3
Interest-rate swap derivatives	72
Total	75
Level 1 [Member]
Assets:
Commodity derivatives	0
Total	0
Liabilities:
Commodity derivatives	0
Interest-rate swap derivatives	0
Total	0
Level 2 [Member]
Assets:
Commodity derivatives	6
Total	6
Liabilities:
Commodity derivatives	3
Interest-rate swap derivatives	72
Total	75
Level 3 [Member]
Assets:
Commodity derivatives	0
Total	0
Liabilities:
Commodity derivatives	0
Interest-rate swap derivatives	0
Total	$ 0