Derivative Instruments and Hedging Activities - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($) Derivative Swap	Dec. 31, 2014 USD ($) Derivative Swap
Derivative Instruments, Gain (Loss) [Line Items]
Reclassification adjustment from accumulated other comprehensive income	$ 1.3
Termination value of derivatives in a net liability position	17.3
Minimum collateral with counterparties	$ 17.2
Minimum
Derivative Instruments, Gain (Loss) [Line Items]
Number of days for commitments to expire, lower limit	30 days
Maximum
Derivative Instruments, Gain (Loss) [Line Items]
Number of days for commitments to expire, higher limit	60 days
Not Designated as Hedging Instrument | Residential mortgage loan commitments
Derivative Instruments, Gain (Loss) [Line Items]
Aggregate notional amount	$ 4.3	$ 3.8
Interest Rate Swaps | Derivative Designated as Cash Flow Hedges | Cash Flow Hedges
Derivative Instruments, Gain (Loss) [Line Items]
Number of outstanding interest rate derivative | Derivative	1	1
Aggregate notional amount	$ 150.0	$ 150.0
Interest Rate Swaps | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Aggregate notional amount	$ 394.3	$ 251.9
Number of interest rate swaps | Swap	52	44
Credit Risk Contract | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Aggregate notional amount	$ 19.4	$ 13.4