Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	$ 209,000,000.0	$ 192,300,000
Included in Condensed Consolidated Statement of Operations	1,200,000	(300,000)
Included in Other Comprehensive Income (Loss)	2,500,000	1,800,000
Purchases	169,800,000	13,200,000
Sales	(21,300,000)	(3,700,000)
Transfers into Level 3	15,000,000.0	1,700,000
Transfers out of Level 3	(100,000)	(13,100,000)
Balance, End of Period	376,100,000	191,900,000
Corporate Bonds and Notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	194,200,000	177,100,000
Included in Condensed Consolidated Statement of Operations	1,200,000	(200,000)
Included in Other Comprehensive Income (Loss)	1,800,000	2,300,000
Purchases	95,000,000.0	13,200,000
Sales	(21,300,000)	(3,700,000)
Transfers into Level 3	5,000,000.0	0
Transfers out of Level 3	(100,000)	(8,200,000)
Balance, End of Period	275,800,000	180,500,000
States and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	1,800,000	100,000
Included in Condensed Consolidated Statement of Operations	0	0
Included in Other Comprehensive Income (Loss)	(100,000)	(400,000)
Purchases	0	0
Sales	0	0
Transfers into Level 3	0	1,700,000
Transfers out of Level 3	0	0
Balance, End of Period	1,700,000	1,400,000
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	4,200,000	7,100,000
Included in Condensed Consolidated Statement of Operations	0	0
Included in Other Comprehensive Income (Loss)	100,000	0
Purchases	0	0
Sales	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	(4,900,000)
Balance, End of Period	4,300,000	2,200,000
Collateralized Loan Obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	0
Included in Condensed Consolidated Statement of Operations	0
Included in Other Comprehensive Income (Loss)	0
Purchases	67,000,000.0
Sales	0
Transfers into Level 3	0
Transfers out of Level 3	0
Balance, End of Period	67,000,000.0
Other Mortgage- and Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	5,000,000.0	5,200,000
Included in Condensed Consolidated Statement of Operations	0	0
Included in Other Comprehensive Income (Loss)	0	(100,000)
Purchases	7,500,000	0
Sales	0	0
Transfers into Level 3	10,000,000.0	0
Transfers out of Level 3	0	0
Balance, End of Period	22,500,000	5,100,000
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	3,800,000	2,800,000
Included in Condensed Consolidated Statement of Operations	0	(100,000)
Included in Other Comprehensive Income (Loss)	700,000	0
Purchases	300,000	0
Sales	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance, End of Period	$ 4,800,000	$ 2,700,000