Interest-bearing Time Deposits in Banks and Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 213,670	$ 240,351	$ 5,280
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	2,896	884	30
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	17,794	139,487	1,211,060
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	31	950	14,139
Available-for-sale securities, continuous unrealized loss position, Fair Value	231,464	379,838	1,216,340
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	2,927	1,834	14,169
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	168,725	65,787	2,807
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	2,717	400	13
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value		326	48,068
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss			523
Available-for-sale securities, continuous unrealized loss position, Fair Value	168,725	66,113	50,875
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	2,717	400	536
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	220
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	6
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value			4,873
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss			15
Available-for-sale securities, continuous unrealized loss position, Fair Value	220		4,873
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	6		15
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	100,004	2,473
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss		306	17
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	8,164	103,983	854,482
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	21	842	11,205
Available-for-sale securities, continuous unrealized loss position, Fair Value	8,165	203,987	856,955
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	21	1,148	11,222
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	44,724	74,560
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	173	178
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	9,630	35,178	303,637
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	10	108	2,396
Available-for-sale securities, continuous unrealized loss position, Fair Value	54,354	109,738	303,637
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 183	$ 286	$ 2,396