Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2011

Fair Value Measurements [Abstract]

Fair value of assets and liabilities measured on recurring basis

	June 30, 2011								December 31, 2010
	Level 1		Level 2		Level 3		Total		Level 1		Level 2		Level 3		Total
	(Millions)
Assets:
Energy derivatives	$	46	$	352	$	3	$	401	$	96	$	475	$	2	$	573
ARO Trust investments (see Note 11)		40		—		—		40		40		—		—		40
Available-for-sale equity securities (see Note 11)		27		—		—		27		—		—		—		—
Total assets	$	113	$	352	$	3	$	468	$	136	$	475	$	2	$	613
Liabilities:
Energy derivatives	$	41	$	173	$	2	$	216	$	78	$	210	$	1	$	289
Total liabilities	$	41	$	173	$	2	$	216	$	78	$	210	$	1	$	289

Fair Value Measurements Using Significant Unobservable Inputs

	Three months ended June 30,						Six months ended June 30,
	2011			2010			2011			2010
	(Millions)
Beginning balance	$	—		$	5		$	1		$	2
Realized and unrealized gains (losses):
Included in income (loss) from continuing operations		3			(1	)		2			(1	)
Included in other comprehensive income (loss)		—			11			(1	)		15
Settlements		(2	)		(1	)		(2	)		(2	)
Transfers into Level 3		—			—			—			—
Transfers out of Level 3		—			—			1			—
Ending balance	$	1		$	14		$	1		$	14
Unrealized gains (losses) included in income (loss) from continuing operations relating to instruments still held at June 30	$	1		$	(1	)	$	—		$	(1	)