Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 481.4	$ 546.6	$ 556.6	$ 581.7
Included in Condensed Consolidated Statement of Operations	1.4	0.4	4.6	1.0
Included in Other Comprehensive Income (Loss)	(1.1)	4.6	(3.0)	0.8
Purchases	420.9	48.2	552.2	149.2
Settlements	(28.0)	(36.7)	(128.4)	(96.6)
Sales	(47.8)	(8.8)	(88.7)	(57.0)
Transfers into Level 3	35.9	6.4	36.2	6.4
Transfers out of Level 3	2.7	19.1	(69.5)	43.9
Balance at End of Period	860.0	541.6	860.0	541.6
Fair Value, Level 2 to level 3 Transfers, Amount			(36.2)	(6.4)
Fair Value, Level 3 to level 1 Transfers, Amount				(3.5)
Fair Value, Level 3 to level 2 Transfers, Amount	2.7		7.7	(40.4)
Bonds and Notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	385.5	396.1	401.5	403.2
Included in Condensed Consolidated Statement of Operations	0.9	(0.8)	2.3	(4.7)
Included in Other Comprehensive Income (Loss)	0.0	1.8	0.1	4.6
Purchases	54.9	40.4	148.6	120.4
Settlements	(11.5)	(31.2)	(77.1)	(75.5)
Sales	(47.8)	(3.7)	(88.7)	(32.0)
Transfers into Level 3	2.0	0.0	2.3	0.0
Transfers out of Level 3	2.7	15.2	7.7	28.6
Balance at End of Period	381.3	387.4	381.3	387.4
States and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	0.0	0.0	0.0	3.8
Included in Condensed Consolidated Statement of Operations	0.0	0.0	0.0	(1.2)
Included in Other Comprehensive Income (Loss)	0.0	0.0	0.0	0.0
Purchases	1.8	0.0	1.8	0.0
Settlements	0.0	0.0	0.0	(2.6)
Sales	0.0	0.0	0.0	0.0
Transfers into Level 3	0.0	1.4	0.0	1.4
Transfers out of Level 3	0.0	0.0	0.0	0.0
Balance at End of Period	1.8	1.4	1.8	1.4
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		0.1	0.1	0.6
Included in Condensed Consolidated Statement of Operations		0.0	(0.1)	0.1
Included in Other Comprehensive Income (Loss)		0.0	0.0	(0.1)
Purchases		0.0	0.0	0.0
Settlements		0.0	0.0	0.5
Sales		0.0	0.0	0.0
Transfers into Level 3		0.0	0.0	0.0
Transfers out of Level 3		0.0	0.0	0.0
Balance at End of Period	0.0	0.1	0.0	0.1
Collateralized Loan Obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	95.9	95.4	93.2	103.5
Included in Condensed Consolidated Statement of Operations	0.5	0.5	2.4	1.4
Included in Other Comprehensive Income (Loss)	(1.3)	0.0	(3.3)	1.7
Purchases	354.2	6.3	391.8	22.7
Settlements	(16.5)	(5.5)	(51.3)	(18.0)
Sales	0.0	4.3	0.0	11.0
Transfers into Level 3	33.9	5.0	33.9	5.0
Transfers out of Level 3	0.0	(3.9)	0.0	(11.8)
Balance at End of Period	466.7	$ 93.5	466.7	$ 93.5
Other Mortgage- and Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	0.0		0.0
Included in Condensed Consolidated Statement of Operations	0.0		0.0
Included in Other Comprehensive Income (Loss)	0.2		0.2
Purchases	10.0		10.0
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 Period	$ 10.2		10.2
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Transfers out of Level 3			$ (61.8)