SUMMARY OF INVESTMENTS - Fair Value of Investments (Details) - USD ($)	Mar. 31, 2026	Dec. 31, 2025
AVAILABLE-FOR-SALE
Amortized Cost	$ 2,271,554,000	$ 2,239,173,000
Gross Unrealized Gain	14,388,000	23,900,000
Gross Unrealized Loss	67,011,000	57,723,000
Allowance for Credit Losses	0	0
Fair Value	2,218,931,000	2,205,350,000
US Treasury and government agencies
AVAILABLE-FOR-SALE
Amortized Cost	104,981,000	109,358,000
Gross Unrealized Gain	216,000	482,000
Gross Unrealized Loss	5,947,000	5,736,000
Allowance for Credit Losses	0	0
Fair Value	99,250,000	104,104,000
States, municipalities and political subdivisions
AVAILABLE-FOR-SALE
Amortized Cost	196,098,000	261,711,000
Gross Unrealized Gain	1,281,000	1,598,000
Gross Unrealized Loss	1,943,000	1,575,000
Allowance for Credit Losses	0	0
Fair Value	195,436,000	261,734,000
Corporate
AVAILABLE-FOR-SALE
Amortized Cost	810,184,000	797,892,000
Gross Unrealized Gain	4,301,000	9,186,000
Gross Unrealized Loss	29,670,000	23,924,000
Allowance for Credit Losses	0	0
Fair Value	784,815,000	783,154,000
Residential mortgage-backed
AVAILABLE-FOR-SALE
Amortized Cost	788,942,000	732,452,000
Gross Unrealized Gain	6,104,000	8,991,000
Gross Unrealized Loss	28,163,000	25,846,000
Allowance for Credit Losses	0	0
Fair Value	766,883,000	715,597,000
Commercial mortgage-backed
AVAILABLE-FOR-SALE
Amortized Cost	146,511,000	143,009,000
Gross Unrealized Gain	1,691,000	2,405,000
Gross Unrealized Loss	59,000	7,000
Allowance for Credit Losses	0	0
Fair Value	148,143,000	145,407,000
Other asset-backed
AVAILABLE-FOR-SALE
Amortized Cost	224,838,000	194,751,000
Gross Unrealized Gain	795,000	1,238,000
Gross Unrealized Loss	1,229,000	635,000
Allowance for Credit Losses	0	0
Fair Value	$ 224,404,000	$ 195,354,000

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
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

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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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Amount excluding accrued interest, of 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/2009/role/commonPracticeRef
-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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