Investment Securities - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2019 USD ($) Investmentsecurities	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Investment Securities [Line Items]
Investment securities sold	$ 125,247,000	$ 293,279,000
Realized Investment Gain	335,000	207,000
Realized Investment Loss	228,000	$ 0
Securities Pledged As Collateral	504,475,000		$ 597,591,000
Residential real estate mortgage-backed securities	$ 867,714,000
Life of mortgage-backed securities	4 years 8 months 12 days
Obligations of U.S. Government Agencies
Investment Securities [Line Items]
Percentage of aggregate depreciation in unrealized losses	0.59%
Asset-backed Securities
Investment Securities [Line Items]
Percentage of aggregate depreciation in unrealized losses	1.49%
Number of available for sale securities in unrealized loss position (in investment securities) | Investmentsecurities	25