Investments - Investment Securities Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 95,065	$ 150,147
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	248	582
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	5,893	1,662
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	85	82
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	100,958	151,809
Held-to-maturity Securities, Gross Unrealized Losses	333	664
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	56,967	110,622
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	172	487
Available-for Sale Fair, Value of Investments with Unrealized Losses, 12 Months or Greater	19,743	18,104
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	3,007	3,007
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	76,710	128,726
Available-for Sale, Gross Unrealized Losses, Total	3,179	3,494
Fair Value of Investments with Unrealized Losses, Less Than 12 months	152,032	260,769
Gross Unrealized Loss, Less Than 12 Months	420	1,069
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	25,636	19,766
Gross Unrealized Losses, 12 Months or Greater	3,092	3,089
Fair Value of Investments with Unrealized Losses, Total	177,668	280,535
Gross Unrealized Losses, Total	3,512	4,158
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		3,385
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss		2
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value		3,385
Held-to-maturity Securities, Gross Unrealized Losses		2
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	76,863	129,788
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	200	520
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	4,280
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	7
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	81,143	129,788
Held-to-maturity Securities, Gross Unrealized Losses	207	520
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	52,767	106,185
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	150	470
Available-for Sale Fair, Value of Investments with Unrealized Losses, 12 Months or Greater	1,028
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	4
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	53,795	106,185
Available-for Sale, Gross Unrealized Losses, Total	154	470
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	18,113	16,896
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	46	58
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	18,113	16,896
Held-to-maturity Securities, Gross Unrealized Losses	46	58
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	4,200	3,810
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	22	6
Available-for Sale Fair, Value of Investments with Unrealized Losses, 12 Months or Greater	9,888	10,347
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	2,167	1,666
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	14,088	14,157
Available-for Sale, Gross Unrealized Losses, Total	2,189	1,672
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	89	78
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	2	2
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	89	78
Held-to-maturity Securities, Gross Unrealized Losses	2	2
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months		627
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months		11
Available-for Sale Fair, Value of Investments with Unrealized Losses, 12 Months or Greater	8,827	7,757
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	836	1,341
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	8,827	8,384
Available-for Sale, Gross Unrealized Losses, Total	836	1,352
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,613	1,662
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	78	82
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	1,613	1,662
Held-to-maturity Securities, Gross Unrealized Losses	$ 78	$ 82