Debt - Additional Information (Detail) In Millions, unless otherwise specified						3 Months Ended			3 Months Ended				3 Months Ended		3 Months Ended
	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Mar. 31, 2014 Canadian Credit Facility Term Loan [Member] CAD	Dec. 31, 2013 Canadian Credit Facility Term Loan [Member] CAD	Mar. 31, 2014 Term Loan [Member] Canadian Credit Facility Term Loan [Member] CAD	Mar. 31, 2014 5.0% Senior Notes Due March 2014 [Member] USD ($)	Mar. 31, 2014 6.375% Senior Notes Due in March 2015 [Member] USD ($)	Mar. 31, 2014 Nontaxable Municipal Bonds [Member] USD ($)	Mar. 31, 2014 U.S. Revolving Credit Facility [Member] USD ($)	Dec. 31, 2013 U.S. Revolving Credit Facility [Member] USD ($)	Mar. 31, 2014 U.S. Revolving Credit Facility [Member] Canadian Credit Facility Term Loan [Member] CAD	Mar. 31, 2014 $2.25 billion Revolving Credit Facility [Member] USD ($)	Mar. 31, 2014 Letter Of Credit Facilities [Member] USD ($)	Mar. 31, 2014 Letter of Credit Facilities [Member] Canadian Credit Facility Term Loan [Member] CAD	Mar. 31, 2014 Canadian Credit Facility [Member] USD ($)	Mar. 31, 2014 Canadian Credit Facility [Member] CAD
Debt Instrument [Line Items]
Debt maturing within twelve months	$ 498
Senior notes	5,933	6,287				350	350
Interest rate of senior notes						5.00%	6.375%
Tax-exempt bonds, current								82
Outstanding borrowing under credit facility									790	420		790	1,300
Credit Facility, aggregate capacity	2,250				500				2,250		150		2,650	50
Tax-exempt bonds subject to re-pricing within next 12 months	874
Short-term borrowings outstanding												420
Debt maturing within twelve months classified as long-term	576											370
Current debt obligations	1,216	726
Maturity date of credit facility									Jul. 26, 2018				Dec. 31, 2016
Letter of credit outstanding revolving credit facility			0	0					884
Unused and available credit capacity	576
Credit facility borrowings				10
Term credit, maturity date			2017-11
Net borrowings (repayments) under credit facility						$ 350			$ 370						$ 23	25