Investment Securities - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2019 USD ($) Investmentsecurities	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Investment Securities [Line Items]
Investment securities sold | $	$ 0	$ 0
Securities Pledged As Collateral | $	587,233,000		$ 597,591,000
Residential real estate mortgage-backed securities | $	$ 1,048,332,000
Life of mortgage-backed securities	5 years 6 months
Obligations of U.S. Government Agencies [Member]
Investment Securities [Line Items]
Available for sale securities in unrealized loss position | Investmentsecurities	93
Percentage of aggregate depreciation in unrealized losses	1.30%
Obligations of States and Political Subdivisions [Member]
Investment Securities [Line Items]
Available for sale securities in unrealized loss position | Investmentsecurities	33
Percentage of aggregate depreciation in unrealized losses	2.00%
Asset-backed Securities [Member]
Investment Securities [Line Items]
Available for sale securities in unrealized loss position | Investmentsecurities	28
Percentage of aggregate depreciation in unrealized losses	1.30%