Interest-bearing Time Deposits in Banks and Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 1,594,398	$ 1,067,714	$ 724,222
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	30,514	5,895	6,066
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	353,912	377,365	118,381
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	11,893	6,608	2,881
Available-for-sale securities, continuous unrealized loss position, Fair Value	1,948,310	1,445,079	842,603
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	42,407	12,503	8,947
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	303	60,329	30,062
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	1	186	21
Available-for-sale securities, continuous unrealized loss position, Fair Value	303	60,329	30,062
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	1	186	21
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	136,950	66,361	110,706
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	1,122	219	1,906
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	43,239	44,938	37,851
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	1,574	717	1,642
Available-for-sale securities, continuous unrealized loss position, Fair Value	180,189	111,299	148,557
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	2,696	936	3,548
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	4,617	224	241
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	76	2	2
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	233	237
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	3	3
Available-for-sale securities, continuous unrealized loss position, Fair Value	4,850	461	241
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	79	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	1,073,327	701,252	422,087
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	21,838	3,988	3,387
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	227,155	239,641	60,410
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	8,880	4,928	1,171
Available-for-sale securities, continuous unrealized loss position, Fair Value	1,300,482	940,893	482,497
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	30,718	8,916	4,558
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	379,201	239,548	150,805
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	7,477	1,500	745
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	83,285	92,549	20,120
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	1,436	960	68
Available-for-sale securities, continuous unrealized loss position, Fair Value	462,486	332,097	170,925
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 8,913	$ 2,460	813
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			10,321
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss			5
Available-for-sale securities, continuous unrealized loss position, Fair Value			10,321
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss			$ 5