Financial Risk Management and Trading Activities - Gross Volumes of Corporation's Derivative Contracts Outstanding (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Interest Rate Swaps [Member]
Derivative [Line Items]
Derivative notional amount	$ 1,300	$ 865
Gross Volumes of the Corporate Risk Management Derivative Contracts Outstanding [Member] | Foreign Exchange [Member]
Derivative [Line Items]
Derivative notional amount	1,197	220
Gross Volumes of the Corporate Risk Management Derivative Contracts Outstanding [Member] | Interest Rate Swaps [Member]
Derivative [Line Items]
Derivative notional amount	1,300	865
Gross Volumes of the Corporate Risk Management Derivative Contracts Outstanding [Member] | Commodity, Primarily Crude Oil (Millions of Barrels) [Member]
Derivative [Line Items]
Absolute volume of derivative commodity contracts	15	9
Gross Volumes of Derivative Contracts Outstanding Relating to Trading Activities [Member] | Foreign Exchange [Member]
Derivative [Line Items]
Derivative notional amount	55	52
Gross Volumes of Derivative Contracts Outstanding Relating to Trading Activities [Member] | Interest Rate Swaps [Member]
Derivative [Line Items]
Derivative notional amount	$ 29
Gross Volumes of Derivative Contracts Outstanding Relating to Trading Activities [Member] | Crude Oil and Refined Petroleum Products [Member]
Derivative [Line Items]
Absolute volume of derivative commodity contracts	1,810	1,815
Gross Volumes of Derivative Contracts Outstanding Relating to Trading Activities [Member] | Natural Gas (Millions of mcf) [Member]
Derivative [Line Items]
Absolute volume of derivative commodity contracts	2,519	2,735
Gross Volumes of Derivative Contracts Outstanding Relating to Trading Activities [Member] | Electricity (Millions of megawatt hours)[Member]
Derivative [Line Items]
Absolute volume of derivative commodity contracts	1	1
Gross Volumes of Derivative Contracts Outstanding Relating to Trading Activities [Member] | Equity Securities (Millions of Shares) [Member]
Derivative [Line Items]
Equity securities	7	11