SUMMARY OF INVESTMENTS - Fair Value of Investments (Details) - USD ($)	Jun. 30, 2026	Dec. 31, 2025
AVAILABLE-FOR-SALE
Amortized Cost	$ 2,340,146,000	$ 2,239,173,000
Gross Unrealized Gain	12,360,000	23,900,000
Gross Unrealized Loss	68,987,000	57,723,000
Allowance for Credit Losses	0	0
Fair Value	2,283,519,000	2,205,350,000
US Treasury and government agencies
AVAILABLE-FOR-SALE
Amortized Cost	99,636,000	109,358,000
Gross Unrealized Gain	88,000	482,000
Gross Unrealized Loss	6,218,000	5,736,000
Allowance for Credit Losses	0	0
Fair Value	93,506,000	104,104,000
States, municipalities and political subdivisions
AVAILABLE-FOR-SALE
Amortized Cost	172,484,000	261,711,000
Gross Unrealized Gain	1,389,000	1,598,000
Gross Unrealized Loss	1,496,000	1,575,000
Allowance for Credit Losses	0	0
Fair Value	172,377,000	261,734,000
Corporate
AVAILABLE-FOR-SALE
Amortized Cost	840,582,000	797,892,000
Gross Unrealized Gain	4,062,000	9,186,000
Gross Unrealized Loss	30,004,000	23,924,000
Allowance for Credit Losses	0	0
Fair Value	814,640,000	783,154,000
Residential mortgage-backed
AVAILABLE-FOR-SALE
Amortized Cost	794,764,000	732,452,000
Gross Unrealized Gain	4,946,000	8,991,000
Gross Unrealized Loss	29,210,000	25,846,000
Allowance for Credit Losses	0	0
Fair Value	770,500,000	715,597,000
Commercial mortgage-backed
AVAILABLE-FOR-SALE
Amortized Cost	169,122,000	143,009,000
Gross Unrealized Gain	1,352,000	2,405,000
Gross Unrealized Loss	174,000	7,000
Allowance for Credit Losses	0	0
Fair Value	170,300,000	145,407,000
Other asset-backed
AVAILABLE-FOR-SALE
Amortized Cost	263,558,000	194,751,000
Gross Unrealized Gain	523,000	1,238,000
Gross Unrealized Loss	1,885,000	635,000
Allowance for Credit Losses	0	0
Fair Value	$ 262,196,000	$ 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/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477657/942-210-S45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478478/942-210-S50-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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-Paragraph 1
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-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

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-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479130/326-30-45-1

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