Investments - Changes in Allowance for Credit Losses by Major Security Type of Investments Classified as Held-to-Maturity (Detail) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	$ 157	$ 274
Provision for credit loss expense (benefit)	15	(102)
Balance	172	172
Amortized cost	541,396	541,396
AAA/AA/A Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	509,037	509,037
Baa/BBB Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	32,338	32,338
B Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	21	21
States and Political Subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	41	42
Provision for credit loss expense (benefit)	4	3
Balance	45	45
Amortized cost	469,445	469,445
States and Political Subdivisions [Member] | AAA/AA/A Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	466,450	466,450
States and Political Subdivisions [Member] | Baa/BBB Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	2,995	2,995
Corporate Bonds [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	111	225
Provision for credit loss expense (benefit)	12	(102)
Balance	123	123
Amortized cost	51,974	51,974
Corporate Bonds [Member] | AAA/AA/A Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	22,647	22,647
Corporate Bonds [Member] | Baa/BBB Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	29,327	29,327
U.S. Agency-Based Mortgage-Backed Securities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	5,060	5,060
U.S. Agency-Based Mortgage-Backed Securities [Member] | AAA/AA/A Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	5,060	5,060
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	14,801	14,801
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] | AAA/AA/A Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	14,801	14,801
Asset-Backed Securities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	5	7
Provision for credit loss expense (benefit)	(1)	(3)
Balance	4	4
Amortized cost	116	116
Asset-Backed Securities [Member] | AAA/AA/A Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	79	79
Asset-Backed Securities [Member] | Baa/BBB Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	16	16
Asset-Backed Securities [Member] | B Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	$ 21	$ 21