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Investments - Cash Flows from Consolidated Investing Activities by Security (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2010
Proceeds from investments sold, matured or repaid:      
Total proceeds from investments sold, matured or repaid $ 6,206,696 $ 1,699,919 $ 30,094
Cost of investments acquired:      
Cost of investments acquired 5,972,715 1,808,999 3,989
Available-for-sale [Member]
      
Proceeds from investments sold, matured or repaid:      
Total proceeds from investments sold, matured or repaid 5,833,423 1,482,195  
Cost of investments acquired:      
Cost of investments acquired 5,640,090 1,285,951  
Held-to-maturity [Member]
      
Proceeds from investments sold, matured or repaid:      
Total proceeds from investments sold, matured or repaid 109,636 101,755 30,094
Cost of investments acquired:      
Cost of investments acquired 68,733 123,428 3,989
Trading [Member]
      
Proceeds from investments sold, matured or repaid:      
Total proceeds from investments sold, matured or repaid 106,074 29,532  
Cost of investments acquired:      
Cost of investments acquired 122,289 332,715  
Derivatives and other [Member]
      
Proceeds from investments sold, matured or repaid:      
Total proceeds from investments sold, matured or repaid 157,563 86,437  
Cost of investments acquired:      
Cost of investments acquired $ 141,603 $ 66,905