Financial Risk Management - Fair Value Measurements - Additional Information (Detail) $ in Millions	Sep. 30, 2015 USD ($)
Derivative [Line Items]
Derivative assets	$ 105
Derivative liabilities	3
Total Long-term debt, Carrying Value	6,552
The fair value of the guarantee, related to HETCO, recorded by the Corporation	11
Commodity
Derivative [Line Items]
Derivative assets	76
Derivative liabilities	3
Interest Rate and Other
Derivative [Line Items]
Derivative assets	29
Fair Value, Inputs, Level 1
Derivative [Line Items]
Derivative liabilities	3
Fair Value, Inputs, Level 2
Derivative [Line Items]
Derivative assets	105
Total Long-term debt, Fair Value	6,935
Fair Value, Inputs, Level 2 | Commodity
Derivative [Line Items]
Derivative assets	76
Fair Value, Inputs, Level 2 | Interest Rate and Other
Derivative [Line Items]
Derivative assets	$ 29