Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 1,155,527	$ 77,449	$ 213,670
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	18,355	179	2,896
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	999	3,156	17,794
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	10	19	31
Available-for-sale securities, continuous unrealized loss position, Fair Value	1,156,526	80,605	231,464
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	18,365	198	2,927
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	533,213	25,214	168,725
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	8,198	79	2,717
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	0	0	0
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	0	0	0
Available-for-sale securities, continuous unrealized loss position, Fair Value	533,213		168,725
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	8,198	79	2,717
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	32,490		220
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	1,539		6
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	0		0
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	0		0
Available-for-sale securities, continuous unrealized loss position, Fair Value	32,490		220
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	1,539		6
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	589,353	36,017	1
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	8,618	96	0
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	999	3,156	8,164
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	10	19	21
Available-for-sale securities, continuous unrealized loss position, Fair Value	590,352	39,173	8,165
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	8,628	115	21
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	471	16,218	44,724
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	0	4	173
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	0	0	9,630
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	0	0	10
Available-for-sale securities, continuous unrealized loss position, Fair Value	471	16,218	54,354
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 0	$ 4	$ 183