Derivative Financial Instruments - Credit-Risk Related Contingent Features (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
S&P Long Term Ratings A [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	$ 0
S&P Long Term Ratings A [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	45
Moody's Long Term Ratings A2 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	45
Moody's Long Term Ratings A3 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	$ 363