Derivative Instruments and Hedging Activities - Additional Information (Detail) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($) derivative swap	Sep. 30, 2018 USD ($) derivative swap	Dec. 31, 2017 USD ($) derivative swap
Derivative Instruments, Gain (Loss) [Line Items]
Reclassification adjustment from accumulated other comprehensive income		$ (1.0)
Length of hedging exposure to variability in future cash flows (months)		33 months
Minimum
Derivative Instruments, Gain (Loss) [Line Items]
Derivative expiration period		30 days
Maximum
Derivative Instruments, Gain (Loss) [Line Items]
Derivative expiration period		60 days
Not Designated as Hedging Instrument | Residential mortgage loan commitments
Derivative Instruments, Gain (Loss) [Line Items]
Aggregate notional amount	$ 6.1	$ 6.1	$ 2.7
Interest Rate Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Aggregate notional amount	500.0	500.0
Interest Rate Swaps | Derivative Designated as Cash Flow Hedges | Cash Flow Hedges
Derivative Instruments, Gain (Loss) [Line Items]
Aggregate notional amount	$ 800.0	$ 800.0	$ 550.0
Number of outstanding interest rate derivatives (derivative) | derivative	8	8	9
Interest Rate Swaps | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Aggregate notional amount	$ 956.8	$ 956.8	$ 800.5
Number of interest rate swaps (swap) | swap	98	98	76
Credit Contract | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Aggregate notional amount	$ 95.5	$ 95.5	$ 80.5
Other Non-interest Income | Interest Rate Swaps | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Gain on cash flow hedge ineffectiveness	$ 2.8