Discontinued Operations - Operating Results for Cross-Border Energy Lease Investments (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) from discontinued operations, net of income taxes	$ 9	$ 9	$ (322)
Cross-Border Energy Lease Investments [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenue from PHI's cross-border energy lease investments			7
Non-cash charge to reduce carrying value of PHI's cross-border energy lease investments			(373)
Total operating revenue			(366)
Income (loss) from operations of discontinued operations, net of income taxes		9	(325)
Net losses associated with the early termination of the cross-border energy lease investments, net of income taxes			(2)
Income (loss) from discontinued operations, net of income taxes		$ 9	$ (327)