Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 165,286	$ 150,147
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	751	582
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	19,556	1,662
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	200	82
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	184,842	151,809
Held-to-maturity Securities, Gross Unrealized Losses	951	664
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	148,851	110,622
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	1,521	487
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	15,449	18,104
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,765	3,007
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	164,300	128,726
Available-for Sale, Gross Unrealized Losses, Total	3,286	3,494
Fair Value of Investments with Unrealized Losses, Less Than 12 months	314,137	260,769
Gross Unrealized Loss, Less Than 12 Months	2,272	1,069
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	35,005	19,766
Gross Unrealized Losses, 12 Months or Greater	1,965	3,089
Fair Value of Investments with Unrealized Losses, Total	349,142	280,535
Gross Unrealized Losses, Total	4,237	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	2,980	3,385
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	4	2
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	2,980	3,385
Held-to-maturity Securities, Gross Unrealized Losses	4	2
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	128,436	129,788
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	687	520
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	18,139
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	123
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	146,575	129,788
Held-to-maturity Securities, Gross Unrealized Losses	810	520
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	141,857	106,185
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	1,475	470
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	4,216
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	11
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	146,073	106,185
Available-for Sale, Gross Unrealized Losses, Total	1,486	470
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	24,068	16,896
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	45	58
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	24,068	16,896
Held-to-maturity Securities, Gross Unrealized Losses	45	58
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	6,560	3,810
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	9	6
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	4,439	10,347
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	198	1,666
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	10,999	14,157
Available-for Sale, Gross Unrealized Losses, Total	207	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	18	78
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss		2
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	28
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	1
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	46	78
Held-to-maturity Securities, Gross Unrealized Losses	1	2
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	434	627
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	37	11
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	6,794	7,757
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,556	1,341
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	7,228	8,384
Available-for Sale, Gross Unrealized Losses, Total	1,593	1,352
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,389	1,662
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	76	82
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	1,389	1,662
Held-to-maturity Securities, Gross Unrealized Losses	76	$ 82
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	9,784
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	15
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	9,784
Held-to-maturity Securities, Gross Unrealized Losses	$ 15