Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 40,830	$ 240,351	$ 17,594
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	97	884	77
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	6,454	139,487	549,604
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	73	950	3,544
Available-for-sale securities, continuous unrealized loss position, Fair Value	47,284	379,838	567,198
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	170	1,834	3,621
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	5,055
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	1
Available-for-sale securities, continuous unrealized loss position, Fair Value	5,055
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	1
Obligations of State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	18,249	65,787
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	50	400
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value		326	9,777
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss			61
Available-for-sale securities, continuous unrealized loss position, Fair Value	18,249	66,113	9,777
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	50	400	61
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	12,590	100,004	17,594
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	45	306	77
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	6,454	103,983	404,946
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	73	842	3,024
Available-for-sale securities, continuous unrealized loss position, Fair Value	19,044	203,987	422,540
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	118	1,148	3,101
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	4,936	74,560
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	1	178
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	0	35,178	134,881
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	0	108	459
Available-for-sale securities, continuous unrealized loss position, Fair Value	4,936	109,738	134,881
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 1	$ 286	$ 459