Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 13,982	$ 166,470
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(32)	(637)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	235,380
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(2,950)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	13,982	401,850
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(32)	(3,587)
US Government Agency Debt Securities Commercial [Domain]
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		54,501
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(201)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		16,365
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		(149)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total		70,866
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		(350)
Residential Mortgage Backed Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	10,777	111,969
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(23)	(436)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	219,015
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(2,801)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	10,777	330,984
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(23)	$ (3,237)
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	998
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	998
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (2)