Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 916,767	$ 77,449	$ 40,830
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	7,279	179	97
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	2,478	3,156	6,454
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	6	19	73
Available-for-sale securities, continuous unrealized loss position, Fair Value	919,245	80,605	47,284
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	7,285	198	170
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 12 Months or Longer, Fair Value
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss
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	170,483	25,214	18,249
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	992	79	50
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss
Available-for-sale securities, continuous unrealized loss position, Fair Value	170,483	25,214	18,249
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	992	79	50
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	33,318
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	581
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss
Available-for-sale securities, continuous unrealized loss position, Fair Value	33,318
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	581
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	712,100	36,017	12,590
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	5,706	96	45
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	2,478	3,156	6,454
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	6	19	73
Available-for-sale securities, continuous unrealized loss position, Fair Value	714,578	39,173	19,044
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	5,712	115	118
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	866	16,218	4,936
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss		4	1
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss
Available-for-sale securities, continuous unrealized loss position, Fair Value	866	16,218	4,936
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss		$ 4	$ 1