Interest-bearing Time Deposits in Banks and Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 47,509	$ 523,075	$ 117,203
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	393	3,758	818
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	63,715	74,777	79,298
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	669	2,083	1,292
Available-for-sale securities, continuous unrealized loss position, Fair Value	111,224	597,852	196,501
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	1,062	5,841	2,110
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	6,804	32,929	84,444
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	23	127	748
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	1,071	1,513	2,570
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	5	7	36
Available-for-sale securities, continuous unrealized loss position, Fair Value	7,875	34,442	87,014
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	28	134	784
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	25,152	231,481	12,917
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	357	1,765	13
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	48,124	63,919	67,164
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	606	1,994	1,204
Available-for-sale securities, continuous unrealized loss position, Fair Value	73,276	295,400	80,081
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	963	3,759	1,217
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	15,553	196,163	19,842
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	13	1,752	57
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	14,520	9,345	9,564
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	58	82	52
Available-for-sale securities, continuous unrealized loss position, Fair Value	30,073	205,508	29,406
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 71	1,834	$ 109
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