Derivative Financial Instruments - Credit-Risk Related Contingent Features (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014
Moody's Short Term Ratings P 1 [Member] | Moody's Long Term Ratings A1 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	$ 0
Moody's Short Term Ratings P 1 [Member] | Moody's Long Term Ratings A2 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	34
Moody's Short Term Ratings P 1 [Member] | Moody's Long Term Ratings A3 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	144
Moody's Short Term Ratings P 2 [Member] | Moody's Long Term Ratings A1 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	1
Moody's Short Term Ratings P 2 [Member] | Moody's Long Term Ratings A2 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	34
Moody's Short Term Ratings P 2 [Member] | Moody's Long Term Ratings A3 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	144
S&P Short Term Ratings A 1 [Member] | S&P Long Term Ratings AA- [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	0
S&P Short Term Ratings A 1 [Member] | S&P Long Term Ratings A [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	0
S&P Short Term Ratings A 1 [Member] | S&P Long Term Ratings A [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	33
S&P Short Term Rating A 1 [Member] | S&P Long Term Ratings AA- [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	5
S&P Short Term Rating A 1 [Member] | S&P Long Term Ratings A [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	5
S&P Short Term Rating A 1 [Member] | S&P Long Term Ratings A [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	$ 38