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Legal & Regulatory Proceedings (Legal and Regulatory Proceedings - Additional Information) (Detail)
$ in Millions
3 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
Mar. 31, 2013
USD ($)
Dec. 31, 2013
Sep. 30, 2013
USD ($)
Sep. 30, 2012
USD ($)
Mar. 31, 2012
USD ($)
Loss Contingencies            
Amount in excess of amounts reserved $ 31.0          
APR Percentage Allegedly Misleading         120.00%  
Number Of Putative Class Actions Filed     4      
Damages Claimed By Plaintiff       $ 50.0 $ 70.0 $ 40.0
Federal Lawsuits            
Loss Contingencies            
Number of merchants requesting exclusion   500        
Class Action Settlement            
Loss Contingencies            
Number of merchants requesting exclusion   8,000        
Percentage of escrow funds returned to defendants   25.00%        
Escrow Deposit   $ 46.0        
Litigation Settlement Amount Awarded From Other Party $ 2.2