Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 485.3	$ 950.7	$ 1,044.2	$ 897.4
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	(2.3)	(4.8)	(5.3)	(7.6)
Included in Other Comprehensive Income (Loss)	19.7	6.8	(15.1)	18.7
Purchases	53.3	80.4	134.7	185.6
Settlements	(0.1)	(16.8)	(0.1)	(28.4)
Sales	(15.5)	(52.7)	(60.3)	(118.6)
Transfers into Level 3	7.4	3.0	7.4	19.5
Transfers out of Level 3	(79.8)	(1.6)	(637.5)	(1.6)
Balance at End of Period	468.0	965.0	468.0	965.0
Bonds and Notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	390.6	411.9	409.1	382.6
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	(2.3)	(5.2)	(5.0)	(7.7)
Included in Other Comprehensive Income (Loss)	18.7	4.8	(6.1)	10.2
Purchases	53.4	60.1	80.6	161.4
Settlements	0.0	(4.6)	0.0	(16.1)
Sales	(15.5)	(49.8)	(33.7)	(115.7)
Transfers into Level 3	7.4	0.0	7.4	2.5
Transfers out of Level 3	0.0	(1.6)	0.0	(1.6)
Balance at End of Period	452.3	415.6	452.3	415.6
States and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	0.7		0.0
Total Gains (Losses):
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)
Balance at End of Period	0.0		0.0
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	6.7		6.7
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	0.0
Included in Other Comprehensive Income (Loss)	(0.1)
Purchases	0.0
Settlements	0.0
Sales	0.0
Transfers into Level 3	0.0
Transfers out of Level 3	(1.2)
Balance at End of Period	5.4		5.4
Collateralized Loan Obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	77.9	528.5	618.2	504.9
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	0.0	0.4	(0.3)	0.1
Included in Other Comprehensive Income (Loss)	0.1	1.7	(9.4)	7.7
Purchases	(0.1)	20.3	53.5	24.2
Settlements	0.0	(12.0)	0.0	(12.0)
Sales	0.0	(2.9)	(26.4)	(2.9)
Transfers into Level 3	0.0	3.0	0.0	17.0
Transfers out of Level 3	(77.9)	0.0	(635.6)	0.0
Balance at End of Period	0.0	539.0	0.0	539.0
Other Mortgage- and Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	9.4	10.3	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)	1.0	0.3	0.4	0.8
Purchases	0.0	0.0	0.0	0.0
Settlements	(0.1)	(0.2)	(0.1)	(0.3)
Sales	0.0	0.0	(0.2)	0.0
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
Balance at End of Period	$ 10.3	$ 10.4	$ 10.3	$ 10.4