Investments - Investment Securities Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 150,147	$ 68,985
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	582	1,188
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,662	2,227
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	82	150
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	151,809	71,212
Held-to-maturity Securities, Gross Unrealized Losses	664	1,338
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	110,622	138,107
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	487	6,191
Available-for Sale Fair, Value of Investments with Unrealized Losses, 12 Months or Greater	18,104	7,525
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	3,007	2,844
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	128,726	145,632
Available-for Sale, Gross Unrealized Losses, Total	3,494	9,035
Fair Value of Investments with Unrealized Losses, Less Than 12 months	260,769	207,092
Gross Unrealized Loss, Less Than 12 Months	1,069	7,379
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	19,766	9,752
Gross Unrealized Losses, 12 Months or Greater	3,089	2,994
Fair Value of Investments with Unrealized Losses, Total	280,535	216,844
Gross Unrealized Losses, Total	4,158	10,373
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	129,788	14,090
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	520	41
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	129,788	14,090
Held-to-maturity Securities, Gross Unrealized Losses	520	41
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	106,185	29,691
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	470	361
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	106,185	29,691
Available-for Sale, Gross Unrealized Losses, Total	470	361
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	16,896	54,895
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	58	1,147
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		311
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss		6
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	16,896	55,206
Held-to-maturity Securities, Gross Unrealized Losses	58	1,153
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	3,810	101,908
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	6	4,798
Available-for Sale Fair, Value of Investments with Unrealized Losses, 12 Months or Greater	10,347	1,753
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,666	342
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	14,157	103,661
Available-for Sale, Gross Unrealized Losses, Total	1,672	5,140
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	78
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	78
Held-to-maturity Securities, Gross Unrealized Losses	2
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	627	1,303
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	11	465
Available-for Sale Fair, Value of Investments with Unrealized Losses, 12 Months or Greater	7,757	5,772
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,341	2,502
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	8,384	7,075
Available-for Sale, Gross Unrealized Losses, Total	1,352	2,967
Equity Securities [Member]
Investment [Line Items]
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months		5,205
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months		567
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total		5,205
Available-for Sale, Gross Unrealized Losses, Total		567
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,662	1,916
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	82	144
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	1,662	1,916
Held-to-maturity Securities, Gross Unrealized Losses	$ 82	$ 144