Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	$ 218,074	$ 206,129
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	972	3,283
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	28,931	7,465
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	403	122
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	247,005	213,594
Held-to-maturity securities, Gross Unrealized Losses, Total	1,375	3,405
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	135,576	173,573
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	520	4,113
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	76,085	16,052
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	2,194	1,454
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	211,661	189,625
Available-for Sale, Gross Unrealized Losses, Total	2,714	5,567
Fair Value of Investments with Unrealized Losses, Less Than 12 months	353,650	379,702
Gross Unrealized Loss, Less Than 12 Months	1,492	7,396
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	105,016	23,517
Gross Unrealized Losses, 12 Months or Greater	2,597	1,576
Fair Value of Investments with Unrealized Losses, Total	458,666	403,219
Gross Unrealized Losses, Total	4,089	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	110,698	157,507
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	654	3,014
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	19,895
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	301
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	130,593	157,507
Held-to-maturity securities, Gross Unrealized Losses, Total	955	3,014
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	23,365	73,505
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	86	2,976
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	19,153	4,523
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	491	239
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	42,518	78,028
Available-for Sale, Gross Unrealized Losses, Total	577	3,215
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	56,425	44,654
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	156	202
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	4,121	6,292
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	11	63
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	60,546	50,946
Held-to-maturity securities, Gross Unrealized Losses, Total	167	265
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	82,795	41,419
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	171	111
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	5,888	7,922
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	41	160
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	88,683	49,341
Available-for Sale, Gross Unrealized Losses, Total	212	271
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	967	1,173
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	15	59
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	967	1,173
Held-to-maturity securities, Gross Unrealized Losses, Total	15	59
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	2,798
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	40
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	2,798
Held-to-maturity securities, Gross Unrealized Losses, Total	40
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	14,686	3,702
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	59	48
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	3,328	3,607
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	740	1,055
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	18,014	7,309
Available-for Sale, Gross Unrealized Losses, Total	799	1,103
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	48,153	3,968
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	122	67
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	3,948
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	76
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	52,101	3,968
Held-to-maturity securities, Gross Unrealized Losses, Total	198	67
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	14,730	54,947
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	204	978
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	47,716
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	922
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	62,446	54,947
Available-for Sale, Gross Unrealized Losses, Total	$ 1,126	$ 978