Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 897.4	$ 556.6
Included in Condensed Consolidated Statement of Operations	(2.8)	2.2
Included in Other Comprehensive Income (Loss)	11.9	(2.1)
Purchases	105.2	74.7
Settlements	(11.6)	(32.1)
Sales	(65.9)	(13.4)
Transfers into Level 3	16.5	0.3
Transfers out of Level 3	0.0	66.8
Balance at End of Period	950.7	519.4
Fair Value, Level 3 to level 2 Transfers, Amount		(5.0)
Bonds and Notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	382.6	401.5
Included in Condensed Consolidated Statement of Operations	(2.5)	1.8
Included in Other Comprehensive Income (Loss)	5.4	(1.6)
Purchases	101.3	25.8
Settlements	(11.5)	(18.1)
Sales	(65.9)	(13.4)
Transfers into Level 3	2.5	0.3
Transfers out of Level 3	0.0	5.0
Balance at End of Period	411.9	391.3
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		0.1
Included in Condensed Consolidated Statement of Operations		(0.1)
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
Balance at End of Period		0.0
Collateralized Loan Obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	504.9	93.2
Included in Condensed Consolidated Statement of Operations	(0.3)	0.5
Included in Other Comprehensive Income (Loss)	6.0	(0.5)
Purchases	3.9	48.9
Settlements	0.0	(14.0)
Sales	0.0	0.0
Transfers into Level 3	14.0	0.0
Transfers out of Level 3	0.0	0.0
Balance at End of Period	528.5	128.1
Other Mortgage- and Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	9.9
Included in Condensed Consolidated Statement of Operations	0.0
Included in Other Comprehensive Income (Loss)	0.5
Purchases	0.0
Settlements	0.1
Sales	0.0
Transfers into Level 3	0.0
Transfers out of Level 3	0.0
Balance at End of Period	$ 10.3
Preferred and Common Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		27.4
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		(27.4)
Balance at End of Period		0.0
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		34.4
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		34.4
Balance at End of Period		$ 0.0