Investment Securities - Additional Information (Detail)	3 Months Ended		6 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) Investmentsecurities	Sep. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Investmentsecurities	Dec. 31, 2017 USD ($)
Investment Securities [Line Items]
Proceeds from sales of available for sale debt securities	$ 293,279,000	$ 25,757,000
Gross realized gains	207,000	$ 961,000
Investment securities sold			$ 0	$ 0
Securities Pledged As Collateral	548,123,000				$ 548,123,000	$ 285,596,000
Residential real estate mortgage-backed securities	$ 1,111,330,000				$ 1,111,330,000
Life of mortgage-backed securities					6 years 1 month 6 days
Obligations of U.S. Government Agencies [Member]
Investment Securities [Line Items]
Available for sale securities in unrealized loss position | Investmentsecurities	171				171
Percentage of aggregate depreciation in unrealized losses	3.70%				3.70%
Obligations of States and Political Subdivisions [Member]
Investment Securities [Line Items]
Available for sale securities in unrealized loss position | Investmentsecurities	132				132
Percentage of aggregate depreciation in unrealized losses	5.00%				5.00%
Corporate Bonds [Member]
Investment Securities [Line Items]
Available for sale securities in unrealized loss position | Investmentsecurities	1				1
Percentage of aggregate depreciation in unrealized losses	0.10%				0.10%
Asset-backed Securities [Member]
Investment Securities [Line Items]
Available for sale securities in unrealized loss position | Investmentsecurities	6				6
Percentage of aggregate depreciation in unrealized losses	0.60%				0.60%