Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 191,300,000	$ 464,800,000	$ 191,300,000	$ 464,800,000	$ 218,900,000	$ 251,400,000	$ 429,200,000	$ 449,200,000
Included in Condensed Consolidated Statement of Operations	(200,000)	1,100,000	(5,900,000)	0
Included in Other Comprehensive Income (Loss)	(8,800,000)	5,400,000	(11,800,000)	400,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	9,900,000	50,100,000	23,600,000	67,900,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	(100,000)	0	(100,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(22,800,000)	(35,100,000)	(65,600,000)	(66,200,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	14,200,000	5,300,000	14,200,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(5,700,000)	0	(5,700,000)	(600,000)
Bonds and Notes
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	177,100,000	409,200,000	177,100,000	409,200,000	205,600,000	236,800,000	414,500,000	433,000,000.0
Included in Condensed Consolidated Statement of Operations	(200,000)	1,100,000	(5,900,000)	0
Included in Other Comprehensive Income (Loss)	(7,700,000)	4,500,000	(9,400,000)	800,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	7,900,000	21,700,000	21,600,000	39,500,000
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	(22,800,000)	(35,100,000)	(65,600,000)	(66,000,000.0)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	2,500,000	5,300,000	2,500,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(5,700,000)	0	(5,700,000)	(600,000)
Redeemable Preferred Stocks
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	6,900,000	6,200,000	6,900,000	6,200,000	5,600,000	6,100,000	5,700,000	6,200,000
Included in Condensed Consolidated Statement of Operations	0	0	0	0
Included in Other Comprehensive Income (Loss)	(700,000)	500,000	(1,200,000)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	2,000,000.0	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		27,800,000		27,800,000			0	0
Included in Condensed Consolidated Statement of Operations		0		0
Included in Other Comprehensive Income (Loss)		100,000		100,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		17,700,000		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		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		10,000,000.0		10,000,000.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		0		0
Other Mortgage- and Asset-backed
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	5,600,000	18,200,000	5,600,000	18,200,000	6,200,000	$ 7,000,000.0	$ 9,000,000.0	10,000,000.0
Included in Condensed Consolidated Statement of Operations	0	0	0	0
Included in Other Comprehensive Income (Loss)	(600,000)	300,000	(1,400,000)	(500,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	9,000,000.0	0	9,000,000.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	(100,000)	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	0	0	0
Equity Securities
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	1,700,000	3,400,000	1,700,000	$ 3,400,000	$ 1,500,000			$ 0
Included in Condensed Consolidated Statement of Operations	0		0
Included in Other Comprehensive Income (Loss)		200,000	200,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	1,700,000	0
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	$ 1,700,000	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	$ 0		$ 0