Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	$ 468.0	$ 965.0	$ 1,044.2	$ 897.4
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	(1.3)	2.1	(6.6)	(5.5)
Included in Other Comprehensive Income (Loss)	5.7	(3.9)	(9.4)	14.8
Purchases	69.5	65.8	204.2	251.4
Settlements	0.0	(15.2)	(0.1)	(43.6)
Sales	(43.9)	(19.0)	(104.2)	(137.6)
Transfers into Level 3			7.4	19.5
Transfers out of Level 3			(637.5)	(1.6)
Balance at End of Period	498.0	994.8	498.0	994.8
Bonds and Notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	452.3	415.6	409.1	382.6
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	(1.3)	1.7	(6.3)	(6.0)
Included in Other Comprehensive Income (Loss)	5.1	(0.8)	(1.0)	9.4
Purchases	69.5	35.1	150.1	196.5
Settlements	0.0	(7.4)	0.0	(23.5)
Sales	(43.8)	(19.0)	(77.5)	(134.7)
Transfers into Level 3			7.4	2.5
Transfers out of Level 3			0.0	(1.6)
Balance at End of Period	481.8	425.2	481.8	425.2
States and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year		0.0	0.0	0.0
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations		0.0	0.0	0.0
Included in Other Comprehensive Income (Loss)		0.0	0.1	0.0
Purchases		1.9	0.6	1.9
Settlements		0.0	0.0	0.0
Sales		0.0	0.0	0.0
Transfers into Level 3			0.0	0.0
Transfers out of Level 3			(0.7)	0.0
Balance at End of Period	0.0	1.9	0.0	1.9
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	5.4		6.7
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	0.0		0.0
Included in Other Comprehensive Income (Loss)	0.7		0.6
Purchases	0.0		0.0
Settlements	0.0		0.0
Sales	(0.1)		(0.1)
Transfers into Level 3			0.0
Transfers out of Level 3			(1.2)
Balance at End of Period	6.0		6.0
Collateralized Loan Obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	0.0	539.0	618.2	504.9
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	0.0	0.4	(0.3)	0.5
Included in Other Comprehensive Income (Loss)	0.0	(3.5)	(9.4)	4.2
Purchases	0.0	8.2	53.5	32.4
Settlements	0.0	(7.6)	0.0	(19.6)
Sales	0.0	0.0	(26.4)	(2.9)
Transfers into Level 3			0.0	17.0
Transfers out of Level 3			(635.6)	0.0
Balance at End of Period	0.0	536.5	0.0	536.5
Other Mortgage- and Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	10.3	10.4	10.2	9.9
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	0.0	0.0	0.0	0.0
Included in Other Comprehensive Income (Loss)	(0.1)	0.4	0.3	1.2
Purchases	0.0	20.6	0.0	20.6
Settlements	0.0	(0.2)	(0.1)	(0.5)
Sales	0.0	0.0	(0.2)	0.0
Transfers into Level 3			0.0	0.0
Transfers out of Level 3			0.0	0.0
Balance at End of Period	$ 10.2	$ 31.2	$ 10.2	$ 31.2