Discontinued Operations (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			3 Months Ended		3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Natural Gas [Member]	Mar. 31, 2013 Discontinued Operations [Member]	Dec. 31, 2012 Discontinued Operations [Member]	Jun. 30, 2013 Subsequent Event [Member] Discontinued Operations [Member]
Derivative [Line Items]
Other energy services receivable			$ 8
Unrealized derivative losses after tax	(2)			(2)
Pre-tax unrealized derivative losses				(4)
Expected gain upon closing of contract						8
Expected gain upon closing of contract, after tax						5
Inventory assets	153	155		1	1
Derivative liabilities				$ 8	$ 12