Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 556.6	$ 581.7
Included in Condensed Consolidated Statement of Operations	2.2	(4.5)
Included in Other Comprehensive Income (Loss)	(2.1)	(2.5)
Purchases	74.7	40.1
Settlements	(32.1)	(22.8)
Sales	(13.4)	(29.9)
Transfers into Level 3	0.3	0.0
Transfers out of Level 3	(66.8)	(10.8)
Balance at End of Period	519.4	551.3
Fair Value, Level 2 to level 3 Transfers, Amount	(0.3)
Fair Value, Level 3 to level 2 Transfers, Amount	(5.0)	(10.8)
Bonds and Notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	401.5	403.2
Included in Condensed Consolidated Statement of Operations	1.8	(3.3)
Included in Other Comprehensive Income (Loss)	(1.6)	1.4
Purchases	25.8	29.8
Settlements	(18.1)	(15.0)
Sales	(13.4)	(22.0)
Transfers into Level 3	0.3
Transfers out of Level 3	(5.0)	(2.9)
Balance at End of Period	391.3	391.2
States and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		3.8
Included in Condensed Consolidated Statement of Operations		(1.2)
Included in Other Comprehensive Income (Loss)		0.0
Purchases		0.0
Settlements		0.0
Sales		0.0
Transfers out of Level 3		0.0
Balance at End of Period		2.6
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	0.1	0.6
Included in Condensed Consolidated Statement of Operations	(0.1)	0.0
Included in Other Comprehensive Income (Loss)	0.0	0.1
Purchases	0.0	0.0
Settlements	0.0	(0.5)
Sales	0.0	0.0
Transfers into Level 3	0.0
Transfers out of Level 3	0.0	0.0
Balance at End of Period	0.0	0.2
Collateralized Loan Obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	93.2	103.5
Included in Condensed Consolidated Statement of Operations	0.5	0.2
Included in Other Comprehensive Income (Loss)	(0.5)	1.9
Purchases	48.9	9.2
Settlements	(14.0)	(7.3)
Sales	0.0	(3.8)
Transfers into Level 3	0.0
Transfers out of Level 3	0.0	(7.9)
Balance at End of Period	128.1	95.8
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	29.7
Included in Condensed Consolidated Statement of Operations	0.0	(0.2)
Included in Other Comprehensive Income (Loss)	0.0	0.7
Purchases	0.0	0.3
Settlements	0.0	0.0
Sales	0.0	(1.4)
Transfers into Level 3	0.0
Transfers out of Level 3	(27.4)	0.0
Balance at End of Period	0.0	29.1
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	34.4	40.9
Included in Condensed Consolidated Statement of Operations	0.0	0.0
Included in Other Comprehensive Income (Loss)	0.0	(6.6)
Purchases	0.0	0.8
Settlements	0.0	0.0
Sales	0.0	(2.7)
Transfers into Level 3	0.0
Transfers out of Level 3	(34.4)	0.0
Balance at End of Period	$ 0.0	$ 32.4