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, 2023 USD ($)	Sep. 30, 2023 USD ($)
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	$ 204	$ 239
Provision for credit loss benefit	(16)	(51)
Balance	188	188
Amortized cost	487,354	487,354
AAA/AA/A Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	454,575	454,575
Baa/BBB Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	32,779	32,779
States and Political Subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	41	40
Provision for credit loss benefit	(4)	(3)
Balance	37	37
Amortized cost	420,017	420,017
States and Political Subdivisions [Member] | AAA/AA/A Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	417,062	417,062
States and Political Subdivisions [Member] | Baa/BBB Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	2,955	2,955
Corporate Bonds [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	161	196
Provision for credit loss benefit	(11)	(46)
Balance	150	150
Amortized cost	52,727	52,727
Corporate Bonds [Member] | AAA/AA/A Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	22,918	22,918
Corporate Bonds [Member] | Baa/BBB Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	29,809	29,809
U.S. Agency-Based Mortgage-Backed Securities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	3,403	3,403
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	3,403	3,403
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	11,162	11,162
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	11,162	11,162
Asset-Backed Securities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	2	3
Provision for credit loss benefit	(1)	(2)
Balance	1	1
Amortized cost	45	45
Asset-Backed Securities [Member] | AAA/AA/A Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	30	30
Asset-Backed Securities [Member] | Baa/BBB Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	$ 15	$ 15