Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	$ 897.4	$ 556.6
Total Gains (Losses):
Included in Consolidated Statement of Income	(6.2)	7.0
Included in Other Comprehensive Income (Loss)	16.8	(18.4)
Purchases	433.0	663.3
Settlements	(101.6)	(166.6)
Sales	(214.3)	(109.5)
Transfers into Level 3	19.1	36.3
Transfers out of Level 3	0.0	(71.3)
Balance at End of Year	1,044.2	897.4
Corporate Bonds and Notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	382.6	401.5
Total Gains (Losses):
Included in Consolidated Statement of Income	(6.8)	4.4
Included in Other Comprehensive Income (Loss)	10.6	(2.2)
Purchases	307.0	201.8
Settlements	(72.9)	(108.1)
Sales	(211.4)	(109.5)
Transfers into Level 3	0.0	2.4
Transfers out of Level 3	0.0	(7.7)
Balance at End of Year	409.1	382.6
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
Total Gains (Losses):
Included in Consolidated Statement of Income		0.0
Included in Other Comprehensive Income (Loss)		0.0
Purchases		1.8
Settlements		0.0
Sales		0.0
Transfers into Level 3		0.0
Transfers out of Level 3		(1.8)
Balance at End of Year		0.0
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	0.0	0.1
Total Gains (Losses):
Included in Consolidated Statement of Income	0.0	(0.1)
Included in Other Comprehensive Income (Loss)	(0.1)	0.0
Purchases	6.8	0.0
Settlements	0.0	0.0
Sales	0.0	0.0
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Balance at End of Year	6.7	0.0
Collateralized Loan Obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	504.9	93.2
Total Gains (Losses):
Included in Consolidated Statement of Income	0.6	2.7
Included in Other Comprehensive Income (Loss)	5.3	(16.3)
Purchases	119.2	449.7
Settlements	(28.0)	(58.3)
Sales	(2.9)	0.0
Transfers into Level 3	19.1	33.9
Transfers out of Level 3	0.0	0.0
Balance at End of Year	618.2	504.9
Other Mortgage- and Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	9.9	0.0
Total Gains (Losses):
Included in Consolidated Statement of Income	0.0	0.0
Included in Other Comprehensive Income (Loss)	1.0	0.1
Purchases	0.0	10.0
Settlements	(0.7)	(0.2)
Sales	0.0	0.0
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Balance at End of Year	10.2	9.9
Preferred and Common Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	0.0	27.4
Total Gains (Losses):
Included in Consolidated Statement of Income		0.0
Included in Other Comprehensive Income (Loss)		0.0
Purchases		0.0
Settlements		0.0
Sales		0.0
Transfers into Level 3		0.0
Transfers out of Level 3		(27.4)
Balance at End of Year		0.0
Other Equity Interests
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	$ 0.0	34.4
Total Gains (Losses):
Included in Consolidated Statement of Income		0.0
Included in Other Comprehensive Income (Loss)		0.0
Purchases		0.0
Settlements		0.0
Sales		0.0
Transfers into Level 3		0.0
Transfers out of Level 3		(34.4)
Balance at End of Year		$ 0.0