Derivatives - Freestanding Derivative Assets and Liabilities Not Designated as Hedges (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Derivatives Fair Value [Line Items]
Other assets	$ 28,360,000	$ 33,182,000
Other liabilities	0	0
Not Designated as Hedging Instrument [Member] | Purchased Certificate of Deposit Option [Member]
Derivatives Fair Value [Line Items]
Other assets	809,000	882,000
Not Designated as Hedging Instrument [Member] | Written Certificate of Deposit Option [Member]
Derivatives Fair Value [Line Items]
Other liabilities	$ 809,000	$ 882,000