Interest-bearing Time Deposits in Banks and Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 1,012,122	$ 523,075
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	15,712	3,758
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	47,699	74,777
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	1,047	2,083
Available-for-sale securities, continuous unrealized loss position, Fair Value	1,059,821	597,852
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	16,759	5,841
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	446,052	32,929
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	9,997	127
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	1,209	1,513
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	16	7
Available-for-sale securities, continuous unrealized loss position, Fair Value	447,261	34,442
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	10,013	134
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	372,331	231,481
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	4,532	1,765
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	33,227	63,919
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	981	1,994
Available-for-sale securities, continuous unrealized loss position, Fair Value	405,558	295,400
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	5,513	3,759
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	193,495	196,163
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	1,180	1,752
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	13,263	9,345
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	50	82
Available-for-sale securities, continuous unrealized loss position, Fair Value	206,758	205,508
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	1,230	1,834
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	244	7,004
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	3	5
Available-for-sale securities, continuous unrealized loss position, Fair Value	244	7,004
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 3	$ 5