Derivative Instruments and Hedging Activities - Additional Information (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($) swap derivative	Dec. 31, 2017 USD ($) swap derivative
Derivative Instruments, Gain (Loss) [Line Items]
Reclassification adjustment from accumulated other comprehensive income	$ 0.5
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	$ 6.0	$ 2.7
Interest Rate Swaps | Derivative Designated as Cash Flow Hedges | Cash Flow Hedges
Derivative Instruments, Gain (Loss) [Line Items]
Number of outstanding interest rate derivatives (derivative) | derivative	13	9
Aggregate notional amount	$ 1,400.0	$ 550.0
Interest Rate Swaps | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Aggregate notional amount	$ 779.0	$ 800.5
Number of interest rate swaps (swap) | swap	82	76
Credit Contract | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Aggregate notional amount	$ 92.6	$ 80.5