Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	$ 1,044.2	$ 897.4
Total Gains (Losses):
Included in Consolidated Statement of Income	(9.3)	(6.2)
Included in Other Comprehensive Income (Loss)	(5.1)	16.8
Purchases	240.2	433.0
Settlements	(0.1)	(101.6)
Sales	(192.1)	(214.3)
Transfers into Level 3	9.0	19.1
Transfers out of Level 3	(637.6)	0.0
Balance at End of Year	449.2	1,044.2
Corporate Bonds and Notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	409.1	382.6
Total Gains (Losses):
Included in Consolidated Statement of Income	(9.0)	(6.8)
Included in Other Comprehensive Income (Loss)	3.2	10.6
Purchases	185.9	307.0
Settlements	0.0	(72.9)
Sales	(165.2)	(211.4)
Transfers into Level 3	9.0	0.0
Transfers out of Level 3	0.0	0.0
Balance at End of Year	433.0	409.1
States and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	0.0
Total Gains (Losses):
Included in Consolidated Statement of Income	0.0
Included in Other Comprehensive Income (Loss)	0.1
Purchases	0.6
Settlements	0.0
Sales	0.0
Transfers into Level 3	0.0
Transfers out of Level 3	(0.7)
Balance at End of Year	0.0	0.0
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	6.7	0.0
Total Gains (Losses):
Included in Consolidated Statement of Income	0.0	0.0
Included in Other Comprehensive Income (Loss)	0.5	(0.1)
Purchases	0.2	6.8
Settlements	0.0	0.0
Sales	0.0	0.0
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	(1.2)	0.0
Balance at End of Year	6.2	6.7
Collateralized Loan Obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	618.2	504.9
Total Gains (Losses):
Included in Consolidated Statement of Income	(0.3)	0.6
Included in Other Comprehensive Income (Loss)	(9.3)	5.3
Purchases	53.5	119.2
Settlements	0.0	(28.0)
Sales	(26.4)	(2.9)
Transfers into Level 3	0.0	19.1
Transfers out of Level 3	(635.7)	0.0
Balance at End of Year	0.0	618.2
Other Mortgage- and Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	10.2	9.9
Total Gains (Losses):
Included in Consolidated Statement of Income	0.0	0.0
Included in Other Comprehensive Income (Loss)	0.4	1.0
Purchases	0.0	0.0
Settlements	(0.1)	(0.7)
Sales	(0.5)	0.0
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Balance at End of Year	$ 10.0	$ 10.2