Interest-bearing Time Deposits in Banks and Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 22,044	$ 523,075	$ 615,230
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	21	3,758	6,807
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	75,257	74,777	77,597
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	627	2,083	2,108
Available-for-sale securities, continuous unrealized loss position, Fair Value	97,301	597,852	692,827
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	648	5,841	8,915
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	5,959	32,929	304,510
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	7	127	5,335
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	745	1,513	2,531
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	3	7	65
Available-for-sale securities, continuous unrealized loss position, Fair Value	6,704	34,442	307,041
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	10	134	5,400
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	16,085	231,481	176,569
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	14	1,765	844
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	60,360	63,919	65,562
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	592	1,994	1,994
Available-for-sale securities, continuous unrealized loss position, Fair Value	76,445	295,400	242,131
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	606	3,759	2,838
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		196,163	134,151
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss		1,752	628
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	14,152	9,345	9,504
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	32	82	49
Available-for-sale securities, continuous unrealized loss position, Fair Value	14,152	205,508	143,655
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 32	1,834	$ 677
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		5,110
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss		2
Available-for-sale securities, continuous unrealized loss position, Fair Value		5,110
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss		2
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		50,388
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss		107
Available-for-sale securities, continuous unrealized loss position, Fair Value		50,388
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss		107
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		7,004
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		7,004
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss		$ 5