Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Jun. 30, 2022	Dec. 31, 2021	Jun. 30, 2021	Dec. 31, 2020
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 233,200,000	$ 235,100,000	$ 233,200,000	$ 235,100,000	$ 191,300,000	$ 251,400,000	$ 464,800,000	$ 449,200,000
Included in Condensed Consolidated Statement of Operations	(6,300,000)	1,200,000	(12,200,000)	1,200,000
Included in Other Comprehensive Income (Loss)	(3,300,000)	4,600,000	(15,100,000)	5,000,000.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	96,200,000	14,600,000	119,800,000	82,500,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0	0	(100,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(62,300,000)	(38,400,000)	(127,900,000)	(104,600,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	17,600,000	0	22,900,000	14,200,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	(211,700,000)	(5,700,000)	(212,300,000)
Corporate Debt Securities [Member]
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	218,700,000	213,600,000	218,700,000	213,600,000	177,100,000	236,800,000	409,200,000	433,000,000.0
Included in Condensed Consolidated Statement of Operations	(6,300,000)	900,000	(12,200,000)	900,000
Included in Other Comprehensive Income (Loss)	(2,800,000)	4,700,000	(12,200,000)	5,500,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	93,500,000	7,400,000	115,100,000	46,900,000
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	(60,400,000)	(28,400,000)	(126,000,000.0)	(94,400,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	17,600,000	0	22,900,000	2,500,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	(180,200,000)	(5,700,000)	(180,800,000)
Redeemable Preferred Stocks
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	6,800,000	6,100,000	6,800,000	6,100,000	6,900,000	6,100,000	6,200,000	6,200,000
Included in Condensed Consolidated Statement of Operations	0	0	0	0
Included in Other Comprehensive Income (Loss)	(100,000)	(100,000)	(1,300,000)	(100,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0	2,000,000.0	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
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		0	0	0
Collateralized Loan Obligations
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		0		0			27,800,000	0
Included in Condensed Consolidated Statement of Operations		0		0
Included in Other Comprehensive Income (Loss)		0		100,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0		17,700,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		(10,000,000.0)		(10,000,000.0)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		0		10,000,000.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		(17,800,000)		(17,800,000)
Other Mortgage- and Asset-backed
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	5,200,000	14,300,000	5,200,000	14,300,000	5,600,000	7,000,000.0	18,200,000	10,000,000.0
Included in Condensed Consolidated Statement of Operations	0	0	0	0
Included in Other Comprehensive Income (Loss)	(400,000)	0	(1,800,000)	(500,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	7,200,000	0	16,200,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0	0	(100,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	(200,000)
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	(11,100,000)	0	(11,100,000)
Equity Securities
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	2,500,000	1,100,000	2,500,000	1,100,000	$ 1,700,000	$ 1,500,000	$ 3,400,000	$ 0
Included in Condensed Consolidated Statement of Operations	0	300,000	0	300,000
Included in Other Comprehensive Income (Loss)	0	0	200,000	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	2,700,000	0	2,700,000	1,700,000
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	(1,900,000)	0	(1,900,000)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	$ 0	0	1,700,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	$ 0		$ 0	$ (2,600,000)