Impairments of Long-Lived Assets - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment charges			$ 123
Write-off of capitalized engineering and design costs			35
Central Group [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment charges			57
National Group [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment charges			5
Southwest Group [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment charges			17
Corporate and other group [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment charges			44
Property and equipment asset [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment charges			$ 109