Financial Risk Management and Trading Activities - Fair Values for Corporation's Net Physical Derivative and Financial Assets and (Liabilities) (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	$ 364	$ 975
Total liabilities measured at fair value on a recurring basis	292	1,004
Long-term debt, including current maturities	5,996	5,798
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	195	241
Total liabilities measured at fair value on a recurring basis	206	113
Level 1 [Member] | Continuing Operations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	162
Total liabilities measured at fair value on a recurring basis	174
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	260	425
Total liabilities measured at fair value on a recurring basis	342	883
Level 2 [Member] | Continuing Operations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	260
Total liabilities measured at fair value on a recurring basis	341
Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	120	497
Total liabilities measured at fair value on a recurring basis	20	285
Level 3 [Member] | Continuing Operations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets measured at fair value on a recurring basis	119
Total liabilities measured at fair value on a recurring basis	$ 18