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Debt - Additional Information (Detail) $ in Millions	1 Months Ended		9 Months Ended
Debt - Additional Information (Detail) $ in Millions	Feb. 28, 2015 USD ($)	Jan. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2015 CAD	Sep. 30, 2015 CAD	Jul. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Debt Instrument [Line Items]
Debt maturing within twelve months			$ 701
Tax-exempt bonds subject to re-pricing within next 12 months			487
Debt maturing within twelve months classified as long-term			973
Current Debt Obligations			215				$ 1,090
Letters of credit outstanding revolving credit facility			982
Senior Notes Maturing In 2015 2017 And 2019 [Member]
Debt Instrument [Line Items]
Early repayment of senior notes		$ 947
Senior Notes [Member]
Debt Instrument [Line Items]
Senior Notes Issued	$ 1,800
Proceeds from notes	$ 1,780
3.125% Senior Notes Due March 1, 2025 [Member]
Debt Instrument [Line Items]
Interest rate of senior notes	3.125%
Debt instrument face amount	$ 600
Maturity Date	Mar. 01, 2025
3.90% Senior Notes Due March 1, 2035 [Member]
Debt Instrument [Line Items]
Interest rate of senior notes	3.90%
Debt instrument face amount	$ 450
Maturity Date	Mar. 01, 2035
4.10% Senior Notes Due March 1, 2045 [Member]
Debt Instrument [Line Items]
Interest rate of senior notes	4.10%
Debt instrument face amount	$ 750
Maturity Date	Mar. 01, 2045
Nontaxable Municipal Bonds [Member]
Debt Instrument [Line Items]
Tax-exempt bonds, current			117
Nontaxable Municipal Bonds [Member] \| Variable Rate [Member]
Debt Instrument [Line Items]
Variable-rate tax-exempt bonds			491
Tax-exempt Bonds [Member]
Debt Instrument [Line Items]
Repayment of tax exempt bonds			50
2.6% Senior Notes Mature in September 2016 [Member]
Debt Instrument [Line Items]
Senior note amount			$ 500
Canadian Credit Facility [Member]
Debt Instrument [Line Items]
Credit Facility, aggregate capacity \| CAD					CAD 150,000,000
Letters of credit outstanding revolving credit facility \| CAD					CAD 0
Term credit, maturity date			2017-11	2017-11
Early Extinguishment of Debt [Member]
Debt Instrument [Line Items]
Net borrowings (repayments) of senior notes charges			$ 430
Early Extinguishment of Debt [Member] \| Senior Notes Maturing In 2015 2017 And 2019 [Member]
Debt Instrument [Line Items]
Net borrowings (repayments) of senior notes charges			122
Early Extinguishment of Debt [Member] \| 7.10% Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Senior note amount			449
Early repayment of senior notes			$ 145
Interest rate of senior notes			7.10%		7.10%
Maturity period			2026	2026
Early Extinguishment of Debt [Member] \| 7.00% Senior Notes Due 2028 [Member]
Debt Instrument [Line Items]
Senior note amount			$ 577
Early repayment of senior notes			$ 182
Interest rate of senior notes			7.00%		7.00%
Maturity period			2028	2028
Early Extinguishment of Debt [Member] \| 7.375% Senior Notes Due 2029 [Member]
Debt Instrument [Line Items]
Senior note amount			$ 223
Early repayment of senior notes			$ 84
Interest rate of senior notes			7.375%		7.375%
Maturity period			2029	2029
Early Extinguishment of Debt [Member] \| 7.75% Senior Notes Due 2032 [Member]
Debt Instrument [Line Items]
Senior note amount			$ 496
Early repayment of senior notes			$ 286
Interest rate of senior notes			7.75%		7.75%
Maturity period			2032	2032
Early Extinguishment of Debt [Member] \| 6.125% Senior Notes Due 2039 [Member]
Debt Instrument [Line Items]
Senior note amount			$ 600
Early repayment of senior notes			$ 326
Interest rate of senior notes			6.125%		6.125%
Maturity period			2039	2039
U.S. Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Credit Facility, aggregate capacity			$ 2,250			$ 2,250
Maturity date of credit facility			Jul. 10, 2020	Jul. 10, 2020
Letters of credit outstanding revolving credit facility			$ 832
Unused and available credit capacity			1,418
Letter of Credit Facilities [Member]
Debt Instrument [Line Items]
Credit Facility, aggregate capacity			$ 2,400
Maturity date of credit facility			Dec. 31, 2018	Dec. 31, 2018
Letter of Credit Facilities [Member] \| Canadian Credit Facility [Member]
Debt Instrument [Line Items]
Credit Facility, aggregate capacity \| CAD					CAD 50,000,000
Canadian Credit Facility and Term Loan [Member]
Debt Instrument [Line Items]
Maturity date of credit facility			Nov. 07, 2017	Nov. 07, 2017
Net borrowings (repayments)			$ 93	CAD 119,000,000
Domestic Line of Credit [Member]
Debt Instrument [Line Items]
Credit Facility, aggregate capacity			$ 2,250