Investments - Investment Securities Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Marketable Securities [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	$ 21,074	$ 48,682
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months	193	99
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	3,311	278,163
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater	13	2,867
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	24,385	326,845
Held-to-Maturity, Gross Unrealized Losses, Total	206	2,966
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	23,573	75,208
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	122	360
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	18,220	218,832
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	43	4,440
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	41,793	294,040
Available-for Sale, Gross Unrealized Losses, Total	165	4,800
Equity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months		18,651
Equity securities, Gross Unrealized Losses, Less Than 12 Months		1,341
Equity securities, Fair Value of Investments with Unrealized Losses, Total		18,651
Equity securities, Gross Unrealized Losses, Total		1,341
Fair Value of Investments with Unrealized Losses, Less Than 12 months	44,647	142,541
Gross Unrealized Loss, Less Than 12 Months	315	1,800
Fair Value of Investments with Unrealized Losses, 12 Months or Greater	21,531	496,995
Gross Unrealized Losses, 12 Months or Greater	56	7,307
Fair Value of Investments with Unrealized Losses, Total	66,178	639,536
Gross Unrealized Losses, Total	371	9,107
Domestic Common Stock [Member]
Marketable Securities [Line Items]
Equity securities, Fair Value of Investments with Unrealized Losses, Less Than 12 Months		18,651
Equity securities, Gross Unrealized Losses, Less Than 12 Months		1,341
Equity securities, Fair Value of Investments with Unrealized Losses, Total		18,651
Equity securities, Gross Unrealized Losses, Total		1,341
States and Political Subdivisions [Member]
Marketable Securities [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months	21,074	28,369
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months	193	59
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, 12 Months or Greater		180,550
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater		2,025
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	21,074	208,919
Held-to-Maturity, Gross Unrealized Losses, Total	193	2,084
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	4,140	16,109
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	26	81
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater		76,255
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater		2,037
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	4,140	92,364
Available-for Sale, Gross Unrealized Losses, Total	26	2,118
Corporate Bonds [Member]
Marketable Securities [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months		17,448
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months		36
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, 12 Months or Greater		48,315
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater		419
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total		65,763
Held-to-Maturity, Gross Unrealized Losses, Total		455
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	6,426	59,099
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	1	279
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater		70,306
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater		654
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	6,426	129,405
Available-for Sale, Gross Unrealized Losses, Total	1	933
U.S. Agency-Based Mortgage-Backed Securities [Member]
Marketable Securities [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, 12 Months or Greater		2,287
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater		80
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total		2,287
Held-to-Maturity, Gross Unrealized Losses, Total		80
Available-for Sale, Fair value of Investments with Unrealized Losses, Less Than 12 Months	13,007
Available-for Sale, Gross Unrealized Losses, Less Than 12 Months	95
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	1,152	12,515
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	1	320
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	14,159	12,515
Available-for Sale, Gross Unrealized Losses, Total	96	320
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Marketable Securities [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Less Than 12 Months		2,865
Held-to-Maturity, Gross Unrealized Losses, Less Than 12 Months		4
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	3,243	46,486
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater	12	335
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	3,243	49,351
Held-to-Maturity, Gross Unrealized Losses, Total	12	339
Available-for Sale, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	17,068	59,756
Available-for Sale, Gross Unrealized Losses, 12 Months or Greater	42	1,429
Available-for Sale, Fair Value of Investments with Unrealized Losses, Total	17,068	59,756
Available-for Sale, Gross Unrealized Losses, Total	42	1,429
Asset-Backed Securities [Member]
Marketable Securities [Line Items]
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, 12 Months or Greater	68	525
Held-to-Maturity, Gross Unrealized Losses, 12 Months or Greater	1	8
Held-to-Maturity, Fair Value of Investments with Unrealized Losses, Total	68	525
Held-to-Maturity, Gross Unrealized Losses, Total	$ 1	$ 8