Interest-bearing Time Deposits in Banks and Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 166,313	$ 165,357	$ 107,564
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	1,197	509	505
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	77,313	204,682	286,782
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	1,146	3,520	8,062
Available-for-sale securities, continuous unrealized loss position, Fair Value	243,626	370,039	394,346
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	2,343	4,029	8,567
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	97,670	15,128	17,871
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	792	60	38
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	2,233	21,249	50,158
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	20	190	539
Available-for-sale securities, continuous unrealized loss position, Fair Value	99,903	36,377	68,029
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	812	250	577
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	62,765	76,350	82,382
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	396	148	450
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	65,614	128,368	134,271
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	1,102	2,710	5,150
Available-for-sale securities, continuous unrealized loss position, Fair Value	128,379	204,718	216,653
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	1,498	2,858	5,600
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	5,878	48,399	6,435
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	9	273	15
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	9,466	55,065	102,353
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	24	620	2,373
Available-for-sale securities, continuous unrealized loss position, Fair Value	15,344	103,464	108,788
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	$ 33	893	2,388
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		25,480	876
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss		28	2
Available-for-sale securities, continuous unrealized loss position, Fair Value		25,480	876
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss		$ 28	$ 2