Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	$ 149,638	$ 206,129
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	838	3,283
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	10,637	7,465
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	205	122
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	160,275	213,594
Held-to-maturity securities, Gross Unrealized Losses, Total	1,043	3,405
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	116,577	173,573
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	1,768	4,113
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	20,307	16,052
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,405	1,454
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	136,884	189,625
Available-for Sale, Gross Unrealized Losses, Total	3,173	5,567
Fair Value of Investments with Unrealized Losses, Less Than 12 months	266,215	379,702
Gross Unrealized Loss, Less Than 12 Months	2,606	7,396
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	30,944	23,517
Gross Unrealized Losses, 12 Months or Greater	1,610	1,576
Fair Value of Investments with Unrealized Losses, Total	297,159	403,219
Gross Unrealized Losses, Total	4,216	8,972
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	79,641	157,507
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	622	3,014
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	3,296
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	155
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	82,937	157,507
Held-to-maturity securities, Gross Unrealized Losses, Total	777	3,014
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	33,677	73,505
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	1,036	2,976
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	8,798	4,523
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	420	239
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	42,475	78,028
Available-for Sale, Gross Unrealized Losses, Total	1,456	3,215
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	46,659	44,654
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	142	202
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	6,242	6,292
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	34	63
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	52,901	50,946
Held-to-maturity securities, Gross Unrealized Losses, Total	176	265
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	22,167	41,419
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	79	111
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	7,919	7,922
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	118	160
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	30,086	49,341
Available-for Sale, Gross Unrealized Losses, Total	197	271
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	658
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	2
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	658
Held-to-maturity securities, Gross Unrealized Losses, Total	2
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	5,510	3,702
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	46	48
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	3,590	3,607
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	867	1,055
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	9,100	7,309
Available-for Sale, Gross Unrealized Losses, Total	913	1,103
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	1,099	1,173
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	16	59
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	1,099	1,173
Held-to-maturity securities, Gross Unrealized Losses, Total	16	59
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	22,680	3,968
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	72	67
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	22,680	3,968
Held-to-maturity securities, Gross Unrealized Losses, Total	72	67
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	55,223	54,947
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	607	978
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	55,223	54,947
Available-for Sale, Gross Unrealized Losses, Total	$ 607	$ 978