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Settlement Guarantee Management - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended
Jun. 30, 2017
Sep. 30, 2016
Settlement Guarantee Management [Abstract]    
Estimated Maximum Settlement Exposure $ 69,600 $ 67,800
Covered settlement exposure $ 3,200 2,900
Estimated probability-weighted value of the guarantee   $ 2