Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 1,812,030	$ 77,449	$ 445,288
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	13,987	179	4,951
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	20,762	3,156	4,916
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	471	19	36
Available-for-sale securities, continuous unrealized loss position, Fair Value	1,832,792	80,605	450,204
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	14,458	198	4,987
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	570,916	25,214	395,729
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	6,036	79	4,875
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	18,711
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	467
Available-for-sale securities, continuous unrealized loss position, Fair Value	589,627	25,214	395,729
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	6,503	79	4,875
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	32,985
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	785
Available-for-sale securities, continuous unrealized loss position, Fair Value	32,985
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	785
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	1,208,129	36,017	49,559
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	7,166	96	76
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	2,051	3,156	4,916
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	4	19	36
Available-for-sale securities, continuous unrealized loss position, Fair Value	1,210,180	39,173	54,475
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 7,170	115	$ 112
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		16,218
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss		4
Available-for-sale securities, continuous unrealized loss position, Fair Value		16,218
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss		$ 4