Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	$ 246,700,000	$ 201,500,000	$ 192,300,000	$ 230,000,000.0
Included in Condensed Consolidated Statement of Operations	100,000	1,700,000	1,800,000	800,000
Included in Other Comprehensive Income (Loss)	2,000,000.0	(2,700,000)	2,000,000.0	(300,000)
Purchases	30,900,000	6,200,000	112,700,000	45,200,000
Sales	(31,800,000)	(14,100,000)	(49,800,000)	(83,400,000)
Transfers into Level 3	5,700,000	5,000,000.0	10,500,000	5,400,000
Transfers out of Level 3	(8,700,000)	0	(24,600,000)	(100,000)
Balance, End of Period	244,900,000	197,600,000	244,900,000	197,600,000
Corporate Bonds and Notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	227,300,000	187,400,000	177,100,000	216,000,000.0
Included in Condensed Consolidated Statement of Operations	300,000	200,000	200,000	700,000
Included in Other Comprehensive Income (Loss)	1,700,000	(2,200,000)	2,300,000	200,000
Purchases	30,900,000	6,100,000	103,500,000	44,000,000.0
Sales	(31,800,000)	(14,100,000)	(48,100,000)	(83,400,000)
Transfers into Level 3	3,900,000	5,000,000.0	7,000,000.0	5,000,000.0
Transfers out of Level 3	(1,900,000)	0	(11,600,000)	(100,000)
Balance, End of Period	230,400,000	182,400,000	230,400,000	182,400,000
States and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	100,000	0	100,000	0
Included in Condensed Consolidated Statement of Operations	0	0	0	0
Included in Other Comprehensive Income (Loss)	(200,000)	0	(600,000)	0
Purchases	0	100,000	0	100,000
Sales	0	0	0	0
Transfers into Level 3	1,800,000	0	3,500,000	0
Transfers out of Level 3	0	0	(1,300,000)	0
Balance, End of Period	1,700,000	100,000	1,700,000	100,000
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	4,100,000	6,700,000	7,100,000	6,800,000
Included in Condensed Consolidated Statement of Operations	0	0	0	0
Included in Other Comprehensive Income (Loss)	200,000	(100,000)	200,000	(200,000)
Purchases	0	0	1,900,000	0
Sales	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	(4,900,000)	0
Balance, End of Period	4,300,000	6,600,000	4,300,000	6,600,000
Collateralized Loan Obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	6,800,000		0
Included in Condensed Consolidated Statement of Operations	0		0
Included in Other Comprehensive Income (Loss)	0		0
Purchases	0		6,800,000
Sales	0		0
Transfers into Level 3	0		0
Transfers out of Level 3	(6,800,000)		(6,800,000)
Balance, End of Period	0		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	5,200,000	5,100,000
Included in Condensed Consolidated Statement of Operations	0	0	0	0
Included in Other Comprehensive Income (Loss)	300,000	(400,000)	100,000	(300,000)
Purchases	0	0	0	0
Sales	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Balance, End of Period	5,300,000	4,800,000	5,300,000	4,800,000
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	3,400,000	2,200,000	2,800,000	2,100,000
Included in Condensed Consolidated Statement of Operations	(200,000)	1,500,000	1,600,000	100,000
Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	0	0	500,000	1,100,000
Sales	0	0	(1,700,000)	0
Transfers into Level 3	0	0	0	400,000
Transfers out of Level 3	0	0	0	0
Balance, End of Period	$ 3,200,000	$ 3,700,000	$ 3,200,000	$ 3,700,000