Interest-bearing Time Deposits in Banks and Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 1,499,165	$ 1,067,714	$ 999,053
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	24,287	5,895	13,394
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	358,148	377,365	45,561
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	10,958	6,608	966
Available-for-sale securities, continuous unrealized loss position, Fair Value	1,857,313	1,445,079	1,044,614
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	35,245	12,503	14,360
Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	50,215	60,329	10,008
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	252	186	2
Available-for-sale securities, continuous unrealized loss position, Fair Value	50,215	60,329	10,008
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	252	186	2
Obligations of States 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	156,791	66,361	259,419
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	1,103	219	7,391
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	43,335	44,938	1,215
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	1,664	717	8
Available-for-sale securities, continuous unrealized loss position, Fair Value	200,126	111,299	260,634
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	2,767	936	7,399
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	9,135	224	9,001
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	51	2	3
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	235	237
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	4	3
Available-for-sale securities, continuous unrealized loss position, Fair Value	9,370	461	9,001
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	55	5	3
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	963,962	701,252	494,836
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	16,825	3,988	4,544
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	224,777	239,641	31,558
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	7,797	4,928	893
Available-for-sale securities, continuous unrealized loss position, Fair Value	1,188,739	940,893	526,394
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	24,622	8,916	5,437
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	319,062	239,548	225,789
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	6,056	1,500	1,454
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	89,801	92,549	12,788
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	1,493	960	65
Available-for-sale securities, continuous unrealized loss position, Fair Value	408,863	332,097	238,577
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 7,549	$ 2,460	$ 1,519