Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 1,044.2	$ 897.4
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	(3.0)	(2.8)
Included in Other Comprehensive Income (Loss)	(34.8)	11.9
Purchases	81.4	105.2
Settlements	0.0	(11.6)
Sales	(44.8)	(65.9)
Transfers into Level 3	0.0	16.5
Transfers out of Level 3	(557.7)	0.0
Balance at End of Period	485.3	950.7
Bonds and Notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	409.1	382.6
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	(2.7)	(2.5)
Included in Other Comprehensive Income (Loss)	(24.8)	5.4
Purchases	27.2	101.3
Settlements	0.0	(11.5)
Sales	(18.2)	(65.9)
Transfers into Level 3	0.0	2.5
Transfers out of Level 3	0.0	0.0
Balance at End of Period	390.6	411.9
States and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	0.0
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	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	0.0
Balance at End of Period	0.7
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	6.7
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	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	0.0
Collateralized Loan Obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	618.2	504.9
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	(0.3)	(0.3)
Included in Other Comprehensive Income (Loss)	(9.5)	6.0
Purchases	53.6	3.9
Settlements	0.0	0.0
Sales	(26.4)	0.0
Transfers into Level 3	0.0	14.0
Transfers out of Level 3	(557.7)	0.0
Balance at End of Period	77.9	528.5
Other Mortgage- and Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	10.2	9.9
Total Gains (Losses):
Included in Condensed Consolidated Statement of Operations	0.0	0.0
Included in Other Comprehensive Income (Loss)	(0.6)	0.5
Purchases	0.0	0.0
Settlements	0.0	(0.1)
Sales	(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	$ 9.4	$ 10.3