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Derivative Financial Instruments - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended 9 Months Ended
Oct. 31, 2007
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Derivative [Line Items]          
Deferred accumulated gain         $ 2,600,000
Deferred accumulated after-tax gain         1,700,000
Weighted Average Strike Rate Outstanding Interest Rate Caps       2.42%  
Approximate credit exposure related to swaps with bank customers   $ 47,200,000   $ 47,200,000  
Aggregate fair value of securities posted as collateral for derivative contracts   22,600,000   22,600,000  
Cash collateral for borrowed securities   21,500,000   21,500,000  
Interest Rate Swaps With Upstream Financial Institution Counterparties [Member]          
Derivative [Line Items]          
Approximate credit exposure related to swaps with bank customers   7,400,000   7,400,000  
Variable-Rate Loans Outstanding [Member]          
Derivative [Line Items]          
Number of Interest Rate Derivatives Held 3        
Notional amount of interest rate swap contracts $ 1,200,000,000        
Principal amount outstanding of variable-rate loans $ 1,200,000,000        
Period for which cash flow hedges were used to hedge monthly interest receipts 84 months        
Interest Income [Member] | Designated as Hedging Instrument [Member] | Variable Rate Loans Interest Rate Swaps Caps Floors [Member]          
Derivative [Line Items]          
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net   $ 0 $ 0 $ 0 $ 0