Derivative Financial Instruments - Credit-Risk Related Contingent Features (Detail) $ in Millions	Sep. 30, 2016 USD ($)
S&P Long Term Rating A plus [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	$ 59
S&P Long Term Rating A [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	65
Moody's Long Term Rating A1 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	59
Moody's Long Term Rating A2 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	$ 65