Summary of Securities with Gross Unrealized Losses (Detail) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
U.S. Treasuries
Schedule Of Available For Sale Securities [Line Items]
Total, Gross Unrealized Losses	$ (5,661)
Obligations of States and Political Subdivisions
Schedule Of Available For Sale Securities [Line Items]
Total, Gross Unrealized Losses	(1,513)
Mortgage Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Total, Gross Unrealized Losses	(4,738)
Asset-backed Securities
Schedule Of Available For Sale Securities [Line Items]
Total, Gross Unrealized Losses	(6,995)
Commercial Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Total, Gross Unrealized Losses	(5,723)
Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Total, Gross Unrealized Losses	(12,237)
Foreign Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Total, Gross Unrealized Losses	(7,237)
Fixed Income Securities
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	731,401	$ 992,284
Less than 12 months, Gross Unrealized Losses	(14,488)	(34,755)
12 months or longer, Fair Value	415,115	212,980
12 months or longer, Gross Unrealized Losses	(29,616)	(19,361)
Total, Fair Value	1,146,516	1,205,264
Total, Gross Unrealized Losses	(44,104)	(54,116)
Fixed Income Securities \| U.S. Treasuries
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	298,152	335,781
Less than 12 months, Gross Unrealized Losses	(4,692)	(7,518)
12 months or longer, Fair Value	12,768	8,322
12 months or longer, Gross Unrealized Losses	(969)	(912)
Total, Fair Value	310,920	344,103
Total, Gross Unrealized Losses	(5,661)	(8,430)
Fixed Income Securities \| Obligations of States and Political Subdivisions
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	16,216	27,772
Less than 12 months, Gross Unrealized Losses	(285)	(1,378)
12 months or longer, Fair Value	15,657	3,778
12 months or longer, Gross Unrealized Losses	(1,228)	(498)
Total, Fair Value	31,873	31,550
Total, Gross Unrealized Losses	(1,513)	(1,876)
Fixed Income Securities \| Mortgage Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	25,678	51,517
Less than 12 months, Gross Unrealized Losses	(1,788)	(4,228)
12 months or longer, Fair Value	25,407	7,860
12 months or longer, Gross Unrealized Losses	(2,950)	(1,381)
Total, Fair Value	51,085	59,377
Total, Gross Unrealized Losses	(4,738)	(5,609)
Fixed Income Securities \| Asset-backed Securities
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	57,761	97,857
Less than 12 months, Gross Unrealized Losses	(1,230)	(3,610)
12 months or longer, Fair Value	94,079	62,689
12 months or longer, Gross Unrealized Losses	(5,765)	(5,541)
Total, Fair Value	151,840	160,546
Total, Gross Unrealized Losses	(6,995)	(9,151)
Fixed Income Securities \| Commercial Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	19,425	67,926
Less than 12 months, Gross Unrealized Losses	(811)	(4,072)
12 months or longer, Fair Value	76,881	27,907
12 months or longer, Gross Unrealized Losses	(4,912)	(2,061)
Total, Fair Value	96,306	95,833
Total, Gross Unrealized Losses	(5,723)	(6,133)
Fixed Income Securities \| Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	201,660	261,123
Less than 12 months, Gross Unrealized Losses	(3,963)	(8,480)
12 months or longer, Fair Value	124,030	71,192
12 months or longer, Gross Unrealized Losses	(8,274)	(6,378)
Total, Fair Value	325,690	332,315
Total, Gross Unrealized Losses	(12,237)	(14,858)
Fixed Income Securities \| Foreign Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Less than 12 months, Fair Value	112,509	150,308
Less than 12 months, Gross Unrealized Losses	(1,719)	(5,469)
12 months or longer, Fair Value	66,293	31,232
12 months or longer, Gross Unrealized Losses	(5,518)	(2,590)
Total, Fair Value	178,802	181,540
Total, Gross Unrealized Losses	$ (7,237)	$ (8,059)

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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