Derivative Financial Instruments - Credit-Risk Related Contingent Features (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013
Moodys Short Term Ratings P 1 [Member] | Moody's, A1 Rating [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	$ 0
Moodys Short Term Ratings P 1 [Member] | Moody's, A2 Rating [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	42
Moodys Short Term Ratings P 1 [Member] | Moody's, A3 Rating [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	141
Moodys Short Term Ratings P 2 [Member] | Moody's, A1 Rating [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	2
Moodys Short Term Ratings P 2 [Member] | Moody's, A2 Rating [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	42
Moodys Short Term Ratings P 2 [Member] | Moody's, A3 Rating [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	141
S And P Short Term Ratings A 1 [Member] | Standard & Poor's, AA- Rating [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	0
S And P Short Term Ratings A 1 [Member] | Standard & Poor's, A+ Rating [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	0
S And P Short Term Ratings A 1 [Member] | Standard & Poor's, A Rating [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	40
S And P Short Term Rating A 1 [Member] | Standard & Poor's, AA- Rating [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	14
S And P Short Term Rating A 1 [Member] | Standard & Poor's, A+ Rating [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	14
S And P Short Term Rating A 1 [Member] | Standard & Poor's, A Rating [Member]
Credit Derivatives [Line Items]
Total Credit Rating Scenario of Derivatives	$ 54