Derivative Financial Instruments - Credit-Risk Related Contingent Features (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Moody's, A1 Rating [Member] | Moodys Short Term Ratings P-1 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	$ 0
Moody's, A1 Rating [Member] | Moodys Short Term Ratings P-2 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	2
Moody's, A2 Rating [Member] | Moodys Short Term Ratings P-1 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	2
Moody's, A2 Rating [Member] | Moodys Short Term Ratings P-2 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	70
Moody's, A3 Rating [Member] | Moodys Short Term Ratings P-1 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	269
Moody's, A3 Rating [Member] | Moodys Short Term Ratings P-2 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	269
Standard & Poor's, AA- Rating [Member] | S And P Short Term Ratings A-1+ [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	0
Standard & Poor's, AA- Rating [Member] | S And P Short Term Rating A-1 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	19
Standard & Poor's, A+ Rating [Member] | S And P Short Term Ratings A-1+ [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	68
Standard & Poor's, A+ Rating [Member] | S And P Short Term Rating A-1 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	19
Standard & Poor's, A Rating [Member] | S And P Short Term Ratings A-1+ [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	68
Standard & Poor's, A Rating [Member] | S And P Short Term Rating A-1 [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	$ 286