Subsequent Event (Narrative) (Detail) (USD $) In Millions, unless otherwise specified		12 Months Ended	3 Months Ended		12 Months Ended	3 Months Ended	12 Months Ended		3 Months Ended
	Dec. 31, 2012	Dec. 31, 2012 PHI [Member]	Mar. 31, 2013 Subsequent Event [Member]	Mar. 31, 2013 Subsequent Event [Member] Minimum [Member]	Mar. 31, 2013 Subsequent Event [Member] Minimum [Member]	Mar. 31, 2013 Subsequent Event [Member] Maximum [Member]	Mar. 31, 2013 Subsequent Event [Member] Maximum [Member]	Mar. 31, 2013 Subsequent Event [Member] PHI [Member]	Mar. 31, 2013 Subsequent Event [Member] PHI [Member] Maximum [Member]
Subsequent Event [Line Items]
After-tax effect of revised lease rerun				$ 355		$ 380
Maximum tax penalty percentage								20.00%	20.00%
Additional federal and state taxes		600	600	170	170	200	200
Potential interest on potential tax liability related to disallowed tax benefits	144		150	50	50	60	60
Additional taxes and related interest				$ 220		$ 260
Estimates partial or complete liquidation cross-boarder energy lease investments, period			1 year