Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 235,100,000	$ 498,000,000.0	$ 235,100,000	$ 498,000,000.0	$ 464,800,000	$ 449,200,000	$ 468,000,000.0	$ 1,044,200,000
Included in Condensed Consolidated Statement of Operations	1,200,000	(1,300,000)	1,200,000	(6,600,000)
Included in Other Comprehensive Income (Loss)	4,600,000	5,700,000	5,000,000.0	(9,400,000)
Purchases	14,600,000	69,500,000	82,500,000	204,200,000
Settlements	0	0	(100,000)	(100,000)
Sales	(38,400,000)	(43,900,000)	(104,600,000)	(104,200,000)
Transfers into Level 3	0	0	14,200,000	7,400,000
Transfers out of Level 3	(211,700,000)	0	(212,300,000)	(637,500,000)
Bonds and Notes
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	213,600,000	481,800,000	213,600,000	481,800,000	409,200,000	433,000,000.0	452,300,000	409,100,000
Included in Condensed Consolidated Statement of Operations	900,000	(1,300,000)	900,000	(6,300,000)
Included in Other Comprehensive Income (Loss)	4,700,000	5,100,000	5,500,000	(1,000,000.0)
Purchases	7,400,000	69,500,000	46,900,000	150,100,000
Settlements	0	0	0	0
Sales	(28,400,000)	(43,800,000)	(94,400,000)	(77,500,000)
Transfers into Level 3	0	0	2,500,000	7,400,000
Transfers out of Level 3	(180,200,000)	0	(180,800,000)	0
States and Political Subdivisions
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		0		0			0	0
Included in Condensed Consolidated Statement of Operations		0		0
Included in Other Comprehensive Income (Loss)		0		100,000
Purchases		0		600,000
Settlements		0		0
Sales		0		0
Transfers into Level 3		0		0
Transfers out of Level 3		0		(700,000)
Redeemable Preferred Stocks
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	6,100,000	6,000,000.0	6,100,000	6,000,000.0	6,200,000	6,200,000	5,400,000	6,700,000
Included in Condensed Consolidated Statement of Operations	0	0	0	0
Included in Other Comprehensive Income (Loss)	(100,000)	700,000	(100,000)	600,000
Purchases	0	0	0	0
Settlements	0	0	0	0
Sales	0	(100,000)	0	(100,000)
Transfers into Level 3		0	0	0
Transfers out of Level 3		0	0	(1,200,000)
Collateralized Loan Obligations
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	0	0	0	0	27,800,000	0	0	618,200,000
Included in Condensed Consolidated Statement of Operations	0	0	0	(300,000)
Included in Other Comprehensive Income (Loss)	0	0	100,000	(9,400,000)
Purchases	0	0	17,700,000	53,500,000
Settlements	0	0	0	0
Sales	(10,000,000.0)	0	(10,000,000.0)	(26,400,000)
Transfers into Level 3	0	0	10,000,000.0	0
Transfers out of Level 3	(17,800,000)	0	(17,800,000)	(635,600,000)
Other Mortgage- and Asset-backed
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	14,300,000	10,200,000	14,300,000	10,200,000	18,200,000	$ 10,000,000.0	$ 10,300,000	$ 10,200,000
Included in Condensed Consolidated Statement of Operations	0	0	0	0
Included in Other Comprehensive Income (Loss)	0	(100,000)	(500,000)	300,000
Purchases	7,200,000	0	16,200,000	0
Settlements	0	0	(100,000)	(100,000)
Sales	0	0	(200,000)	(200,000)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(11,100,000)	$ 0	(11,100,000)	$ 0
Equity Securities
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	1,100,000		1,100,000		$ 3,400,000
Included in Condensed Consolidated Statement of Operations	300,000		300,000
Purchases	0		1,700,000
Transfers into Level 3	0		1,700,000
Transfers out of Level 3	$ (2,600,000)		$ (2,600,000)