Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012
Fair Value, Measurements, Nonrecurring | Other Operating Expense
State line evaluation, impairment charge	$ 55
State line evaluation, impairment charge after tax	39
Fair Value, Measurements, Nonrecurring | Other Operating Expense | State Line
State line evaluation, impairment charge	1
Level 3 | Commodity Transactions
Commodity derivatives categorized as Level 3 fair value measurements, net asset	$ 61