Derivative Financial Instruments - Credit-Risk Related Contingent Features (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015
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	47
Moody's Long Term Ratings A2 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	47
Moody's Long Term Ratings A3 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	$ 414