Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022	Dec. 31, 2021
Total Losses:
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 197,600,000	$ 233,200,000	$ 197,600,000	$ 233,200,000	$ 201,500,000	$ 230,000,000.0	$ 191,300,000	$ 251,400,000
Included in Condensed Consolidated Statement of Operations	1,700,000	(6,300,000)	800,000	(12,200,000)
Included in Other Comprehensive Income (Loss)	(2,700,000)	(3,300,000)	(300,000)	(15,100,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	6,200,000	96,200,000	45,200,000	119,800,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(14,100,000)	(62,300,000)	(83,400,000)	(127,900,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	5,000,000.0	17,600,000	5,400,000	22,900,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0	(100,000)	(5,700,000)
Corporate Debt Securities [Member]
Total Losses:
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	182,400,000	218,700,000	182,400,000	218,700,000	187,400,000	216,000,000.0	177,100,000	236,800,000
Included in Condensed Consolidated Statement of Operations	200,000	(6,300,000)	700,000	(12,200,000)
Included in Other Comprehensive Income (Loss)	(2,200,000)	(2,800,000)	200,000	(12,200,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	6,100,000	93,500,000	44,000,000.0	115,100,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(14,100,000)	(60,400,000)	(83,400,000)	(126,000,000.0)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	5,000,000.0	17,600,000	5,000,000.0	22,900,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0	(100,000)	(5,700,000)
States and Political Subdivisions
Total Losses:
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	100,000		100,000			0
Included in Condensed Consolidated Statement of Operations	0		0
Included in Other Comprehensive Income (Loss)	0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	100,000		100,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0		0
Redeemable Preferred Stocks
Total Losses:
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	6,600,000	6,800,000	6,600,000	6,800,000	6,700,000	6,800,000	6,900,000	6,100,000
Included in Condensed Consolidated Statement of Operations	0	0	0	0
Included in Other Comprehensive Income (Loss)	(100,000)	(100,000)	(200,000)	(1,300,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0	0	2,000,000.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		0	0	0
Other Mortgage- and Asset-backed
Total Losses:
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	4,800,000	5,200,000	4,800,000	5,200,000	5,200,000	5,100,000	5,600,000	7,000,000.0
Included in Condensed Consolidated Statement of Operations	0	0	0	0
Included in Other Comprehensive Income (Loss)	(400,000)	(400,000)	(300,000)	(1,800,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0	0	0
Equity Securities
Total Losses:
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	3,700,000	2,500,000	3,700,000	2,500,000	$ 2,200,000	$ 2,100,000	$ 1,700,000	$ 1,500,000
Included in Condensed Consolidated Statement of Operations	1,500,000	0	100,000	0
Included in Other Comprehensive Income (Loss)	0	0	0	200,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	2,700,000	1,100,000	2,700,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	(1,900,000)	0	(1,900,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	$ 0	400,000	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	$ 0		$ 0	$ 0