Interest-bearing Time Deposits in Banks and Securities - Continuous Unrealized-Loss Position of Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 117,203	$ 165,357
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	818	509
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	79,298	204,682
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	1,292	3,520
Available-for-sale securities, continuous unrealized loss position, Fair Value	196,501	370,039
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	2,110	4,029
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	84,444	15,128
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	748	60
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	2,570	21,249
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	36	190
Available-for-sale securities, continuous unrealized loss position, Fair Value	87,014	36,377
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	784	250
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	12,917	76,350
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	13	148
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	67,164	128,368
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	1,204	2,710
Available-for-sale securities, continuous unrealized loss position, Fair Value	80,081	204,718
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	1,217	2,858
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	19,842	48,399
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	57	273
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Fair Value	9,564	55,065
Available-for-sale securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	52	620
Available-for-sale securities, continuous unrealized loss position, Fair Value	29,406	103,464
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss	109	893
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
Available-for-sale securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss		28
Available-for-sale securities, continuous unrealized loss position, Fair Value		25,480
Available-for-sale securities, continuous unrealized loss position, Unrealized Loss		$ 28