Interest-bearing Time Deposits in Banks and Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 1,431,590	$ 1,067,714	$ 495,548
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	29,069	5,895	2,621
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	673,794	377,365	198,307
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	25,392	6,608	3,691
Available-for-sale securities, continuous unrealized loss position, Fair Value	2,105,384	1,445,079	693,855
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	54,461	12,503	6,312
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	302	60,329	45,050
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	1	186	30
Available-for-sale securities, continuous unrealized loss position, Fair Value	302	60,329	45,050
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	1	186	30
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	184,265	66,361	54,983
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	2,165	219	309
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	44,750	44,938	45,217
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	1,932	717	1,285
Available-for-sale securities, continuous unrealized loss position, Fair Value	229,015	111,299	100,200
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	4,097	936	1,594
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	4,399	224
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	110	2
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	449	237	240
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	11	3	2
Available-for-sale securities, continuous unrealized loss position, Fair Value	4,848	461	240
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	121	5	2
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	962,320	701,252	225,369
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	20,850	3,988	1,531
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	461,227	239,641	131,849
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	19,058	4,928	2,266
Available-for-sale securities, continuous unrealized loss position, Fair Value	1,423,547	940,893	357,218
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	39,908	8,916	3,797
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	270,375	239,548	170,146
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	5,908	1,500	751
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	167,368	92,549	21,001
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	4,391	960	138
Available-for-sale securities, continuous unrealized loss position, Fair Value	437,743	332,097	191,147
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	10,299	$ 2,460	$ 889
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	9,929
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	35
Available-for-sale securities, continuous unrealized loss position, Fair Value	9,929
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 35