Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 206,129	$ 165,286
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	3,283	751
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	7,465	19,556
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	122	200
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	213,594	184,842
Held-to-maturity Securities, Gross Unrealized Losses	3,405	951
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	173,573	148,851
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	4,113	1,521
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	16,052	15,449
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,454	1,765
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	189,625	164,300
Available-for Sale, Gross Unrealized Losses, Total	5,567	3,286
Fair Value of Investments with Unrealized Losses, Less Than 12 months	379,702	314,137
Gross Unrealized Loss, Less Than 12 Months	7,396	2,272
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	23,517	35,005
Gross Unrealized Losses, 12 Months or Greater	1,576	1,965
Fair Value of Investments with Unrealized Losses, Total	403,219	349,142
Gross Unrealized Losses, Total	8,972	4,237
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	157,507	24,068
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	3,014	45
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	157,507	24,068
Held-to-maturity Securities, Gross Unrealized Losses	3,014	45
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	73,505	6,560
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	2,976	9
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	4,523	4,439
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	239	198
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	78,028	10,999
Available-for Sale, Gross Unrealized Losses, Total	3,215	207
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	44,654	128,436
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	202	687
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	6,292	18,139
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	63	123
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	50,946	146,575
Held-to-maturity Securities, Gross Unrealized Losses	265	810
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	41,419	141,857
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	111	1,475
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	7,922	4,216
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	160	11
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	49,341	146,073
Available-for Sale, Gross Unrealized Losses, Total	271	1,486
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,173	1,389
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	59	76
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	1,173	1,389
Held-to-maturity Securities, Gross Unrealized Losses	59	76
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
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
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
Held-to-maturity Securities, Gross Unrealized Losses		1
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	3,702	434
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	48	37
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	3,607	6,794
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,055	1,556
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	7,309	7,228
Available-for Sale, Gross Unrealized Losses, Total	1,103	1,593
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	3,968	2,980
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	67	4
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	3,968	2,980
Held-to-maturity Securities, Gross Unrealized Losses	67	$ 4
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	54,947
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	978
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	54,947
Available-for Sale, Gross Unrealized Losses, Total	$ 978