Quarterly report pursuant to Section 13 or 15(d)

Financing Receivables and Investments - Analysis of Financing Receivables and Investments by Type of Obligor and Credit Quality (Parenthetical) (Detail)

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Financing Receivables and Investments - Analysis of Financing Receivables and Investments by Type of Obligor and Credit Quality (Parenthetical) (Detail) (USD $)
 3 Months Ended
Mar. 31, 2014
Transactions
Dec. 31, 2013
Financing Receivable, Recorded Investment [Line Items]    
Allowance for credit losses $ 11,000,000 $ 11,000,000
Number of transactions 13  
Financial receivable outstanding, Average Remaining Balance 13,800,000  
Total aggregate remaining balance 4,600,000  
Maximum [Member]
    
Financing Receivable, Recorded Investment [Line Items]    
Financial receivable outstanding, Average Remaining Balance 1,000,000  
Berkshire Hathaway Inc [Member]
    
Financing Receivable, Recorded Investment [Line Items]    
Debt security, carrying value 37,000,000  
Debt security, maturity 2035  
Exelon Corporation [Member]
    
Financing Receivable, Recorded Investment [Line Items]    
Debt security, carrying value 34,400,000  
Debt security, maturity 2033  
NRG Energy [Member]
    
Financing Receivable, Recorded Investment [Line Items]    
Allowance for credit losses 11,000,000  
Senior debt investment $ 15,100,000