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Settlement Guarantee Management - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended
Dec. 31, 2016
Sep. 30, 2015
Sep. 30, 2016
Settlement Guarantee Management [Abstract]      
Estimated Maximum Settlement Exposure $ 68,700 $ 67,800  
Covered settlement exposure 3,000   $ 2,900
Estimated probability-weighted value of the guarantee $ 2   $ 2