Interest-bearing Time Deposits in Banks and Securities - Continuous Unrealized-Loss Position of Available-for-Sale and Held-to-Maturity Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Available For Sale And Held To Maturity Securities Continuous Unrealized Loss Position [Line Items]
Available-for-sale and held-to-maturity securities, continuous unrealized loss position Less than 12 Months, Fair Value	$ 165,357	$ 653,523
Available-for-sale and held-to-maturity securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	509	22,213
Available-for-sale and held-to-maturity securities, continuous unrealized loss position 12 Months or Longer, Fair Value	204,682	9,777
Available-for-sale and held-to-maturity securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	3,520	645
Available-for-sale and held-to-maturity securities, continuous unrealized loss position, Fair Value	370,039	663,300
Available-for-sale and held-to-maturity securities, continuous unrealized loss position, Unrealized Loss	4,029	22,858
Obligations of U.S. Government Sponsored-Enterprises and Agencies [Member]
Schedule Of Available For Sale And Held To Maturity Securities Continuous Unrealized Loss Position [Line Items]
Available-for-sale and held-to-maturity securities, continuous unrealized loss position Less than 12 Months, Fair Value	25,480
Available-for-sale and held-to-maturity securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	28
Available-for-sale and held-to-maturity securities, continuous unrealized loss position, Fair Value	25,480
Available-for-sale and held-to-maturity securities, continuous unrealized loss position, Unrealized Loss	28
Obligations of State and Political Subdivisions [Member]
Schedule Of Available For Sale And Held To Maturity Securities Continuous Unrealized Loss Position [Line Items]
Available-for-sale and held-to-maturity securities, continuous unrealized loss position Less than 12 Months, Fair Value	15,128	316,394
Available-for-sale and held-to-maturity securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	60	10,973
Available-for-sale and held-to-maturity securities, continuous unrealized loss position 12 Months or Longer, Fair Value	21,249	4,153
Available-for-sale and held-to-maturity securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	190	346
Available-for-sale and held-to-maturity securities, continuous unrealized loss position, Fair Value	36,377	320,547
Available-for-sale and held-to-maturity securities, continuous unrealized loss position, Unrealized Loss	250	11,319
Residential Mortgage-Backed Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities Continuous Unrealized Loss Position [Line Items]
Available-for-sale and held-to-maturity securities, continuous unrealized loss position Less than 12 Months, Fair Value	76,350	228,423
Available-for-sale and held-to-maturity securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	148	7,623
Available-for-sale and held-to-maturity securities, continuous unrealized loss position 12 Months or Longer, Fair Value	128,368	5,624
Available-for-sale and held-to-maturity securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	2,710	299
Available-for-sale and held-to-maturity securities, continuous unrealized loss position, Fair Value	204,718	234,047
Available-for-sale and held-to-maturity securities, continuous unrealized loss position, Unrealized Loss	2,858	7,922
Commercial Mortgage-Backed Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities Continuous Unrealized Loss Position [Line Items]
Available-for-sale and held-to-maturity securities, continuous unrealized loss position Less than 12 Months, Fair Value	48,399	108,706
Available-for-sale and held-to-maturity securities, continuous unrealized loss position Less than 12 Months, Unrealized Loss	273	3,617
Available-for-sale and held-to-maturity securities, continuous unrealized loss position 12 Months or Longer, Fair Value	55,065
Available-for-sale and held-to-maturity securities, continuous unrealized loss position 12 Months or Longer, Unrealized Loss	620
Available-for-sale and held-to-maturity securities, continuous unrealized loss position, Fair Value	103,464	108,706
Available-for-sale and held-to-maturity securities, continuous unrealized loss position, Unrealized Loss	$ 893	$ 3,617