Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	$ 134,588	$ 206,129
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	435	3,283
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	30,699	7,465
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	442	122
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	165,287	213,594
Held-to-maturity securities, Gross Unrealized Losses, Total	877	3,405
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	88,271	173,573
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	705	4,113
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	38,557	16,052
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,938	1,454
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	126,828	189,625
Available-for Sale, Gross Unrealized Losses, Total	2,643	5,567
Fair Value of Investments with Unrealized Losses, Less Than 12 months	222,859	379,702
Gross Unrealized Loss, Less Than 12 Months	1,140	7,396
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	69,256	23,517
Gross Unrealized Losses, 12 Months or Greater	2,380	1,576
Fair Value of Investments with Unrealized Losses, Total	292,115	403,219
Gross Unrealized Losses, Total	3,520	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	61,279	157,507
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	205	3,014
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	22,658
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	368
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	83,937	157,507
Held-to-maturity securities, Gross Unrealized Losses, Total	573	3,014
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	16,373	73,505
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	101	2,976
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	26,213	4,523
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1,142	239
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	42,586	78,028
Available-for Sale, Gross Unrealized Losses, Total	1,243	3,215
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	46,074	44,654
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	183	202
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	3,041	6,292
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	9	63
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	49,115	50,946
Held-to-maturity securities, Gross Unrealized Losses, Total	192	265
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	15,357	41,419
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	35	111
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	4,845	7,922
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	19	160
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	20,202	49,341
Available-for Sale, Gross Unrealized Losses, Total	54	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	2,993
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	2
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	2,993
Held-to-maturity securities, Gross Unrealized Losses, Total	2
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	5,359	3,702
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	19	48
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	3,497	3,607
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	729	1,055
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	8,856	7,309
Available-for Sale, Gross Unrealized Losses, Total	748	1,103
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	1,023	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	1,023	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	24,242	3,968
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	45	67
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	3,977
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	49
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	28,219	3,968
Held-to-maturity securities, Gross Unrealized Losses, Total	94	67
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	51,182	54,947
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	550	978
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	4,002
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	48
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	55,184	54,947
Available-for Sale, Gross Unrealized Losses, Total	$ 598	$ 978