Summary of Investments - Fair Value of Investments (Details) - USD ($)	Mar. 31, 2025	Dec. 31, 2024
AVAILABLE-FOR-SALE
Amortized Cost	$ 1,997,276,000	$ 1,961,531,000
Gross Unrealized Gain	10,261,000	3,640,000
Gross Unrealized Loss	76,446,000	96,840,000
Allowance for Credit Losses	0	0
Fair Value	1,931,091,000	1,868,331,000
US Treasury and government agencies
AVAILABLE-FOR-SALE
Amortized Cost	125,187,000	126,402,000
Gross Unrealized Gain	415,000	153,000
Gross Unrealized Loss	7,208,000	9,255,000
Allowance for Credit Losses	0	0
Fair Value	118,394,000	117,301,000
States, municipalities and political subdivisions
AVAILABLE-FOR-SALE
Amortized Cost	267,816,000	252,936,000
Gross Unrealized Gain	354,000	52,000
Gross Unrealized Loss	4,739,000	5,084,000
Allowance for Credit Losses	0	0
Fair Value	263,431,000	247,904,000
Corporate
AVAILABLE-FOR-SALE
Amortized Cost	705,122,000	728,662,000
Gross Unrealized Gain	3,342,000	1,354,000
Gross Unrealized Loss	32,271,000	40,633,000
Allowance for Credit Losses	0	0
Fair Value	676,193,000	689,382,000
Residential mortgage-backed
AVAILABLE-FOR-SALE
Amortized Cost	632,776,000	623,431,000
Gross Unrealized Gain	4,201,000	864,000
Gross Unrealized Loss	31,227,000	40,884,000
Allowance for Credit Losses	0	0
Fair Value	605,750,000	583,411,000
Commercial mortgage-backed
AVAILABLE-FOR-SALE
Amortized Cost	129,682,000	102,975,000
Gross Unrealized Gain	1,139,000	624,000
Gross Unrealized Loss	272,000	44,000
Allowance for Credit Losses	0	0
Fair Value	130,549,000	103,554,000
Other asset-backed
AVAILABLE-FOR-SALE
Amortized Cost	136,693,000	127,125,000
Gross Unrealized Gain	810,000	593,000
Gross Unrealized Loss	729,000	940,000
Allowance for Credit Losses	0	0
Fair Value	$ 136,774,000	$ 126,779,000

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
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Amount excluding accrued interest, of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479130/326-30-45-1

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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