Investment Securities - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($) Investmentsecurities	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Investment Securities [Line Items]
Investment securities sold	$ 0	$ 0
Investment securities carrying value	415,542,000		$ 285,596,000
Residential real estate mortgage-backed securities	$ 1,114,767,000
Life of mortgage-backed securities	6 years 1 month 6 days
Obligations of U.S. Government Corporations and Agencies [Member]
Investment Securities [Line Items]
Debt securities | Investmentsecurities	128
Percentage of aggregate depreciation in unrealized losses	2.45%
Obligations of States and Political Subdivisions [Member]
Investment Securities [Line Items]
Debt securities | Investmentsecurities	94
Percentage of aggregate depreciation in unrealized losses	3.16%