Summary of Securities with Gross Unrealized Losses (Detail) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
U.S. Treasuries
Schedule Of Available For Sale Securities [Line Items]
Total, Gross Unrealized Losses	$ (8,430)
Obligations of States and Political Subdivisions
Schedule Of Available For Sale Securities [Line Items]
Total, Gross Unrealized Losses	(1,876)
Mortgage Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Total, Gross Unrealized Losses	(5,609)
Asset-backed Securities
Schedule Of Available For Sale Securities [Line Items]
Total, Gross Unrealized Losses	(9,151)
Commercial Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Total, Gross Unrealized Losses	(6,133)
Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Total, Gross Unrealized Losses	(14,858)
Foreign Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Total, Gross Unrealized Losses	(8,059)
Fixed Income Securities
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	992,284	$ 627,112
Less than 12 months, Gross Unrealized Losses	(34,755)	(6,997)
12 months or longer, Fair Value	212,980	18,972
12 months or longer, Gross Unrealized Losses	(19,361)	(634)
Total, Fair Value	1,205,264	646,084
Total, Gross Unrealized Losses	(54,116)	(7,631)
Fixed Income Securities \| U.S. Treasuries
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	335,781	114,894
Less than 12 months, Gross Unrealized Losses	(7,518)	(390)
12 months or longer, Fair Value	8,322	970
12 months or longer, Gross Unrealized Losses	(912)	(29)
Total, Fair Value	344,103	115,864
Total, Gross Unrealized Losses	(8,430)	(419)
Fixed Income Securities \| Agency Obligations
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value		5,380
Less than 12 months, Gross Unrealized Losses		(68)
12 months or longer, Fair Value		0
12 months or longer, Gross Unrealized Losses		0
Total, Fair Value		5,380
Total, Gross Unrealized Losses		(68)
Fixed Income Securities \| Obligations of States and Political Subdivisions
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	27,772	13,346
Less than 12 months, Gross Unrealized Losses	(1,378)	(301)
12 months or longer, Fair Value	3,778	0
12 months or longer, Gross Unrealized Losses	(498)	0
Total, Fair Value	31,550	13,346
Total, Gross Unrealized Losses	(1,876)	(301)
Fixed Income Securities \| Mortgage Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	51,517	143,674
Less than 12 months, Gross Unrealized Losses	(4,228)	(2,222)
12 months or longer, Fair Value	7,860	3,009
12 months or longer, Gross Unrealized Losses	(1,381)	(62)
Total, Fair Value	59,377	146,683
Total, Gross Unrealized Losses	(5,609)	(2,284)
Fixed Income Securities \| Asset-backed Securities
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	97,857	102,309
Less than 12 months, Gross Unrealized Losses	(3,610)	(703)
12 months or longer, Fair Value	62,689	10,662
12 months or longer, Gross Unrealized Losses	(5,541)	(271)
Total, Fair Value	160,546	112,971
Total, Gross Unrealized Losses	(9,151)	(974)
Fixed Income Securities \| Commercial Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	67,926	50,448
Less than 12 months, Gross Unrealized Losses	(4,072)	(466)
12 months or longer, Fair Value	27,907	1,286
12 months or longer, Gross Unrealized Losses	(2,061)	(161)
Total, Fair Value	95,833	51,734
Total, Gross Unrealized Losses	(6,133)	(627)
Fixed Income Securities \| Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	261,123	129,146
Less than 12 months, Gross Unrealized Losses	(8,480)	(1,954)
12 months or longer, Fair Value	71,192	2,633
12 months or longer, Gross Unrealized Losses	(6,378)	(100)
Total, Fair Value	332,315	131,779
Total, Gross Unrealized Losses	(14,858)	(2,054)
Fixed Income Securities \| Foreign Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	150,308	67,915
Less than 12 months, Gross Unrealized Losses	(5,469)	(893)
12 months or longer, Fair Value	31,232	412
12 months or longer, Gross Unrealized Losses	(2,590)	(11)
Total, Fair Value	181,540	68,327
Total, Gross Unrealized Losses	$ (8,059)	$ (904)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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