Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	$ 6,038	$ 48,682
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	19	99
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	37,881	278,163
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	78	2,867
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	43,919	326,845
Held-to-maturity securities, Gross Unrealized Losses, Total	97	2,966
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months		75,208
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months		360
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	79,062	218,832
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	336	4,440
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	79,062	294,040
Available-for Sale, Gross Unrealized Losses, Total	336	4,800
Fair Value of Investments with Unrealized Losses, Less Than 12 months	6,038	123,890
Gross Unrealized Loss, Less Than 12 Months	19	459
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	116,943	496,995
Gross Unrealized Losses, 12 Months or Greater	414	7,307
Fair Value of Investments with Unrealized Losses, Total	122,981	620,885
Gross Unrealized Losses, Total	433	7,766
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	6,038	28,369
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months	19	59
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	11,835	180,550
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	17	2,025
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	17,873	208,919
Held-to-maturity securities, Gross Unrealized Losses, Total	36	2,084
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months		16,109
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months		81
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	7,008	76,255
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	15	2,037
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	7,008	92,364
Available-for Sale, Gross Unrealized Losses, Total	15	2,118
Corporate Bonds [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months		17,448
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months		36
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	15,306	48,315
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	17	419
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	15,306	65,763
Held-to-maturity securities, Gross Unrealized Losses, Total	17	455
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months		59,099
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months		279
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	22,346	70,306
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	39	654
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	22,346	129,405
Available-for Sale, Gross Unrealized Losses, Total	39	933
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	2,201	2,287
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	12	80
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	2,201	2,287
Held-to-maturity securities, Gross Unrealized Losses, Total	12	80
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	10,588	12,515
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	58	320
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	10,588	12,515
Available-for Sale, Gross Unrealized Losses, Total	58	320
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		2,865
Held-to-maturity securities, Gross Unrealized Losses, Less Than 12 Months		4
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	8,362	46,486
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	29	335
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	8,362	49,351
Held-to-maturity securities, Gross Unrealized Losses, Total	29	339
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	39,120	59,756
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	224	1,429
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	39,120	59,756
Available-for Sale, Gross Unrealized Losses, Total	224	1,429
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	177	525
Held-to-maturity securities, Gross Unrealized Losses, 12 Months or Greater	3	8
Held-to-maturity securities, Fair Value of Investments with Unrealized Losses, Total	177	525
Held-to-maturity securities, Gross Unrealized Losses, Total	$ 3	$ 8