Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	$ 376,100,000	$ 191,900,000	$ 209,000,000.0	$ 192,300,000
Included in Condensed Consolidated Statement of Operations	2,200,000	2,000,000.0	3,400,000	1,700,000
Included in Other Comprehensive Income (Loss)	100,000	(1,800,000)	2,600,000	0
Purchases	87,400,000	68,600,000	257,200,000	81,800,000
Sales	(61,000,000.0)	(14,300,000)	(82,300,000)	(18,000,000.0)
Transfers into Level 3	0	3,100,000	15,000,000.0	4,800,000
Transfers out of Level 3	(58,600,000)	(2,800,000)	(58,700,000)	(15,900,000)
Balance, End of Period	346,200,000	246,700,000	346,200,000	246,700,000
Corporate Bonds and Notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	275,800,000	180,500,000	194,200,000	177,100,000
Included in Condensed Consolidated Statement of Operations	300,000	100,000	1,500,000	(100,000)
Included in Other Comprehensive Income (Loss)	900,000	(1,700,000)	2,700,000	600,000
Purchases	33,900,000	59,400,000	128,900,000	72,600,000
Sales	(49,200,000)	(12,600,000)	(70,500,000)	(16,300,000)
Transfers into Level 3	0	3,100,000	5,000,000.0	3,100,000
Transfers out of Level 3	(100,000)	(1,500,000)	(200,000)	(9,700,000)
Balance, End of Period	261,600,000	227,300,000	261,600,000	227,300,000
States and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	1,700,000	1,400,000	1,800,000	100,000
Included in Condensed Consolidated Statement of Operations	0	0	0	0
Included in Other Comprehensive Income (Loss)	0	0	(100,000)	(400,000)
Purchases	0	0	0	0
Sales	0	0	0	0
Transfers into Level 3	0	0	0	1,700,000
Transfers out of Level 3	(1,500,000)	(1,300,000)	(1,500,000)	(1,300,000)
Balance, End of Period	200,000	100,000	200,000	100,000
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	4,300,000	2,200,000	4,200,000	7,100,000
Included in Condensed Consolidated Statement of Operations	0	0	0	0
Included in Other Comprehensive Income (Loss)	0	0	100,000	0
Purchases	0	1,900,000	0	1,900,000
Sales	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	(4,900,000)
Balance, End of Period	4,300,000	4,100,000	4,300,000	4,100,000
Collateralized Loan Obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	67,000,000.0	0	0	0
Included in Condensed Consolidated Statement of Operations	0	0	0	0
Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	52,800,000	6,800,000	119,800,000	6,800,000
Sales	(10,000,000.0)	0	(10,000,000.0)	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(57,000,000.0)	0	(57,000,000.0)	0
Balance, End of Period	52,800,000	6,800,000	52,800,000	6,800,000
Other Mortgage- and Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	22,500,000	5,100,000	5,000,000.0	5,200,000
Included in Condensed Consolidated Statement of Operations	0	0	0	0
Included in Other Comprehensive Income (Loss)	(100,000)	(100,000)	(100,000)	(200,000)
Purchases	200,000	0	7,700,000	0
Sales	(100,000)	0	(100,000)	0
Transfers into Level 3	0	0	10,000,000.0	0
Transfers out of Level 3	0	0	0	0
Balance, End of Period	22,500,000	5,000,000.0	22,500,000	5,000,000.0
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning of Period	4,800,000	2,700,000	3,800,000	2,800,000
Included in Condensed Consolidated Statement of Operations	1,900,000	1,900,000	1,900,000	1,800,000
Included in Other Comprehensive Income (Loss)	(700,000)	0	0	0
Purchases	500,000	500,000	800,000	500,000
Sales	(1,700,000)	(1,700,000)	(1,700,000)	(1,700,000)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Balance, End of Period	$ 4,800,000	$ 3,400,000	$ 4,800,000	$ 3,400,000