Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 464.8	$ 468.0	$ 464.8	$ 468.0	$ 429.2	$ 449.2	$ 485.3	$ 1,044.2
Included in Condensed Consolidated Statement of Operations	1.1	(2.3)	0.0	(5.3)
Included in Other Comprehensive Income (Loss)	5.4	19.7	0.4	(15.1)
Purchases	50.1	53.3	67.9	134.7
Settlements	(0.1)	(0.1)	(0.1)	(0.1)
Sales	(35.1)	(15.5)	(66.2)	(60.3)
Transfers into Level 3	14.2	7.4	14.2	7.4
Transfers out of Level 3	0.0	(79.8)	(0.6)	(637.5)
Bonds and Notes
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	409.2	452.3	409.2	452.3	414.5	433.0	390.6	409.1
Included in Condensed Consolidated Statement of Operations	1.1	(2.3)	0.0	(5.0)
Included in Other Comprehensive Income (Loss)	4.5	18.7	0.8	(6.1)
Purchases	21.7	53.4	39.5	80.6
Settlements	0.0	0.0	0.0	0.0
Sales	(35.1)	(15.5)	(66.0)	(33.7)
Transfers into Level 3	2.5	7.4	2.5	7.4
Transfers out of Level 3	0.0	0.0	(0.6)	0.0
States and Political Subdivisions
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		0.0		0.0			0.7	0.0
Included in Condensed Consolidated Statement of Operations		0.0		0.0
Included in Other Comprehensive Income (Loss)		0.0		0.1
Purchases		0.0		0.6
Settlements		0.0		0.0
Sales		0.0		0.0
Transfers into Level 3		0.0		0.0
Transfers out of Level 3		(0.7)		(0.7)
Redeemable Preferred Stocks
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	6.2	5.4	6.2	5.4	5.7	6.2	6.7	6.7
Included in Condensed Consolidated Statement of Operations	0.0	0.0	0.0	0.0
Included in Other Comprehensive Income (Loss)	0.5	(0.1)	0.0	(0.1)
Purchases	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	0.0
Transfers into Level 3		0.0	0.0	0.0
Transfers out of Level 3		(1.2)	0.0	(1.2)
Collateralized Loan Obligations
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	27.8	0.0	27.8	0.0		0.0	77.9	618.2
Included in Condensed Consolidated Statement of Operations	0.0	0.0	0.0	(0.3)
Included in Other Comprehensive Income (Loss)	0.1	0.1	0.1	(9.4)
Purchases	17.7	(0.1)	17.7	53.5
Settlements	0.0	0.0	0.0	0.0
Sales	0.0	0.0	0.0	(26.4)
Transfers into Level 3	10.0	0.0	10.0	0.0
Transfers out of Level 3	0.0	(77.9)	0.0	(635.6)
Other Mortgage- and Asset-backed
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	18.2	10.3	18.2	10.3	$ 9.0	$ 10.0	$ 9.4	$ 10.2
Included in Condensed Consolidated Statement of Operations	0.0	0.0	0.0	0.0
Included in Other Comprehensive Income (Loss)	0.3	1.0	(0.5)	0.4
Purchases	9.0	0.0	9.0	0.0
Settlements	(0.1)	(0.1)	(0.1)	(0.1)
Sales	0.0	0.0	(0.2)	(0.2)
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	$ 0.0	0.0	$ 0.0
Equity Securities
Total Gains (Losses):
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 3.4		3.4
Purchases			1.7
Transfers into Level 3			$ 1.7