Fair Value Measurements (Details) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Assets:
Energy derivatives	$ 401	$ 573
Liabilities:
Energy derivatives	216	289
Fair Value, Measurements
Assets:
Energy derivatives	401	573
ARO Trust investments	40	40
Available-for-sale equity securities	27	0
Total Assets	468	613
Liabilities:
Energy derivatives	216	289
Total Liabilities	216	289
Fair Value, Measurements | Level 1
Assets:
Energy derivatives	46	96
ARO Trust investments	40	40
Available-for-sale equity securities	27	0
Total Assets	113	136
Liabilities:
Energy derivatives	41	78
Total Liabilities	41	78
Fair Value, Measurements | Level 2
Assets:
Energy derivatives	352	475
ARO Trust investments	0	0
Available-for-sale equity securities	0	0
Total Assets	352	475
Liabilities:
Energy derivatives	173	210
Total Liabilities	173	210
Fair Value, Measurements | Level 3
Assets:
Energy derivatives	3	2
ARO Trust investments	0	0
Available-for-sale equity securities	0	0
Total Assets	3	2
Liabilities:
Energy derivatives	2	1
Total Liabilities	$ 2	$ 1