Quarterly report pursuant to Section 13 or 15(d)

Our Portfolio - Analysis of Portfolio by Type of Obligor and Credit Quality (Parenthetical) (Detail)

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Our Portfolio - Analysis of Portfolio by Type of Obligor and Credit Quality (Parenthetical) (Detail)
$ in Millions
 9 Months Ended
Sep. 30, 2015
USD ($)
Transactions
Financing Receivable, Recorded Investment [Line Items]  
Number of transactions | Transactions 78
Financial receivable outstanding, Average Remaining Balance $ 11
Total aggregate remaining balance 25
Investment Grade By Independent Rating Agency [Member]  
Financing Receivable, Recorded Investment [Line Items]  
Total financing receivable 51
U.S. Federal Government [Member]  
Financing Receivable, Recorded Investment [Line Items]  
Total financing receivable 269
State, Local, Institutions [Member]  
Financing Receivable, Recorded Investment [Line Items]  
Total financing receivable 100
Maximum [Member]  
Financing Receivable, Recorded Investment [Line Items]  
Financial receivable outstanding, Average Remaining Balance 1
Commercial Investment Grade [Member]  
Financing Receivable, Recorded Investment [Line Items]  
Financial receivable outstanding, Average Remaining Balance 10
Commercial Investment Grade [Member] | Residential Solar Loan [Member]  
Financing Receivable, Recorded Investment [Line Items]  
Total financing receivable $ 164