Debt - Narrative (Details)			1 Months Ended	9 Months Ended	12 Months Ended
	Apr. 28, 2023 USD ($)	May 05, 2021 USD ($)	Nov. 30, 2023 USD ($)	Sep. 30, 2023	Dec. 31, 2023 USD ($)
Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Construction loan	$ 125,000,000
Delayed term loan period	5 years
Outstanding borrowings					$ 250,000,000
Additional borrowing capacity					50,000,000
Outstanding borrowings capacity including accordion feature					$ 300,000,000
Maximum allowable consolidated funded indebtedness ratio (percent)					0.35
Minimum net worth required					$ 912,000,000
Minimum liquidity required					$ 25,000,000
Line of Credit | Revolving Credit Facility | Federal Funds Rate
Debt Instrument [Line Items]
Basis spread on variable rate for debt					0.50%
Line of Credit | Revolving Credit Facility | SOFR
Debt Instrument [Line Items]
Basis spread on variable rate for debt					1.00%
Line of Credit | Revolving Credit Facility | Minimum
Debt Instrument [Line Items]
Commitment fee percentages (percent)					0.20%
Line of Credit | Revolving Credit Facility | Maximum
Debt Instrument [Line Items]
Commitment fee percentages (percent)					0.45%
Senior notes due 2023 | Senior Notes
Debt Instrument [Line Items]
Repaid outstanding balance			$ 250,000,000
Contribution Certificates | NORCAL Group
Debt Instrument [Line Items]
Principal amount		$ 191,000,000
Debt instruments		175,000,000
Premium paid on derivative for right to receive cash payments		$ 16,000,000
Debt instrument term		10 years
Revolving Credit Agreement | Line of Credit
Debt Instrument [Line Items]
Basis spread on variable rate for debt				5.30%	2.10%
Revolving Credit Agreement | Line of Credit | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate for debt					0.00%
Revolving Credit Agreement | Line of Credit | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate for debt					2.375%