Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 159,384	$ 206,129
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	2,156	3,283
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	7,377	7,465
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	107	122
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	166,761	213,594
Held-to-maturity Securities, Gross Unrealized Losses	2,263	3,405
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	148,413	173,573
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	3,522	4,113
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	15,963	16,052
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,182	1,454
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	164,376	189,625
Available-for Sale, Gross Unrealized Losses, Total	4,704	5,567
Fair Value of Investments with Unrealized Losses, Less Than 12 months	307,797	379,702
Gross Unrealized Loss, Less Than 12 Months	5,678	7,396
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	23,340	23,517
Gross Unrealized Losses, 12 Months or Greater	1,289	1,576
Fair Value of Investments with Unrealized Losses, Total	331,137	403,219
Gross Unrealized Losses, Total	6,967	8,972
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	106,993	157,507
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	1,895	3,014
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	106,993	157,507
Held-to-maturity Securities, Gross Unrealized Losses	1,895	3,014
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	60,871	73,505
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	2,595	2,976
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	4,374	4,523
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	199	239
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	65,245	78,028
Available-for Sale, Gross Unrealized Losses, Total	2,794	3,215
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	46,021	44,654
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	199	202
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	6,258	6,292
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	57	63
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	52,279	50,946
Held-to-maturity Securities, Gross Unrealized Losses	256	265
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	28,915	41,419
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	58	111
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	7,952	7,922
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	107	160
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	36,867	49,341
Available-for Sale, Gross Unrealized Losses, Total	165	271
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,119	1,173
Held-to-maturity Securities, Continuous Unrealized Loss Position Twelve Months Or Longer, Accumulated Loss	50	59
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	1,119	1,173
Held-to-maturity Securities, Gross Unrealized Losses	50	59
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	3,583	3,702
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	35	48
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	3,637	3,607
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	876	1,055
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	7,220	7,309
Available-for Sale, Gross Unrealized Losses, Total	911	1,103
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	6,370	3,968
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than Twelve Months, Accumulated Loss	62	67
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	6,370	3,968
Held-to-maturity Securities, Gross Unrealized Losses	62	67
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	55,044	54,947
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	834	978
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	55,044	54,947
Available-for Sale, Gross Unrealized Losses, Total	$ 834	$ 978