Commitments and Contingencies - Additional Information (Detail)	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($) institution instrument	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) instrument	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) institution transaction instrument	Dec. 31, 2017 USD ($) transaction instrument	Dec. 31, 2016 USD ($)
Other Commitments [Line Items]
Commitments to sell residential mortgages	$ 32,400,000				$ 28,800,000				$ 32,400,000	$ 28,800,000
Provision for losses at the time of sale	$ 3,306,000	$ 3,716,000	$ 2,498,000	$ 3,650,000	$ 4,063,000	$ 2,896,000	$ 1,843,000	$ 2,162,000	$ 13,170,000	$ 10,964,000	$ 12,986,000
Number of unrelated financial institutions | institution	5								5
Derivative transaction held for guarantee | instrument	136				134				136	134
Aggregate notional amount	$ 167,933,000								$ 167,933,000
Aggregate fair value of swaps to customers	300,000				$ 3,300,000				$ 300,000	$ 3,300,000
Number of customers in paying position to third party | transaction									50	80
Amount of swap liability in paying positions to third party	4,600,000				5,400,000				$ 4,600,000	$ 5,400,000
Reserves for swap guarantees	0				0				0	0
Interest Rate Swap
Other Commitments [Line Items]
Aggregate notional amount	$ 581,500,000				$ 561,800,000				581,500,000	$ 561,800,000
Secondary Market Loan Sales
Other Commitments [Line Items]
Provision for losses at the time of sale									$ 0
Number of loans repurchased | transaction									0	0
Minimum
Other Commitments [Line Items]
Closing period of residential mortgage commitments									1 month
Notional amount maturity period									1 year	1 year
Maximum
Other Commitments [Line Items]
Closing period of residential mortgage commitments									6 months
Notional amount maturity period									11 years	10 years
Real Estate
Other Commitments [Line Items]
Commitments of lending operations									$ 308,900,000
Commitments to Extend Credit
Other Commitments [Line Items]
Commitments of lending operations									1,300,000,000	$ 0
Consumer
Other Commitments [Line Items]
Commitments of lending operations									$ 330,900,000