Investment Securities - Schedule of Securities with Gross Unrealized Losses Aggregated by Investment Category (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Gross Unrealized Losses
Less Than Twelve Months	$ (235)	$ (2,278)
Twelve Months or More	(88)	(2,195)
Total	(323)	(4,473)
Estimated Fair Value
Less Than Twelve Months	33,571	511,891
Twelve Months or More	4,168	283,176
Total	37,739	795,067
Gross Unrealized Losses
Less than twelve months	0	(52)
Over twelve months	(7)	(187)
Total	(7)	(239)
Estimated Fair Value
Less than twelve months	46	6,308
Over twelve months	1,973	9,074
Total	2,019	15,382
Obligations of state and political subdivisions
Gross Unrealized Losses
Less Than Twelve Months		0
Twelve Months or More		0
Total		0
Estimated Fair Value
Less Than Twelve Months		590
Twelve Months or More		0
Total		590
Mortgage-backed securities: Residential agency
Gross Unrealized Losses
Less Than Twelve Months	(70)	(1,893)
Twelve Months or More	0	(1,943)
Total	(70)	(3,836)
Estimated Fair Value
Less Than Twelve Months	8,684	441,332
Twelve Months or More	0	268,383
Total	8,684	709,715
Gross Unrealized Losses
Less than twelve months	0	(52)
Over twelve months	(7)	(187)
Total	(7)	(239)
Estimated Fair Value
Less than twelve months	46	6,308
Over twelve months	1,973	9,074
Total	2,019	15,382
Mortgage-backed securities: Commercial agency
Gross Unrealized Losses
Less Than Twelve Months	(13)	(252)
Twelve Months or More	0	(152)
Total	(13)	(404)
Estimated Fair Value
Less Than Twelve Months	5,039	56,728
Twelve Months or More	0	10,301
Total	5,039	67,029
Other securities
Gross Unrealized Losses
Less Than Twelve Months	(152)	(133)
Twelve Months or More	(88)	(100)
Total	(240)	(233)
Estimated Fair Value
Less Than Twelve Months	19,848	13,241
Twelve Months or More	4,168	4,492
Total	$ 24,016	$ 17,733