Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 546.6	$ 646.8	$ 581.7	$ 606.8
Included in Condensed Consolidated Statement of Operations	0.4	(5.1)	1.0	(15.8)
Included in Other Comprehensive Income (Loss)	4.6	(0.9)	0.8	5.1
Purchases	48.2	74.2	149.2	183.8
Settlements	(36.7)	(25.9)	(96.6)	(40.8)
Sales	(8.8)	(39.0)	(57.0)	(89.0)
Transfers into Level 3	6.4	4.1	6.4	4.1
Transfers out of Level 3	(19.1)	(25.9)	(43.9)	(25.9)
Balance at End of Period	541.6	628.3	541.6	628.3
Fair Value, Level 2 to level 3 Transfers, Amount	(6.4)	(4.1)	(6.4)	(4.1)
Fair Value, Level 3 to level 1 Transfers, Amount			(3.5)
Fair Value, Level 3 to level 2 Transfers, Amount	(19.1)	(25.9)	(40.4)	(25.9)
Corporate Bonds and Notes [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	396.1	459.4	403.2	436.3
Included in Condensed Consolidated Statement of Operations	(0.8)	(7.0)	(4.7)	(15.8)
Included in Other Comprehensive Income (Loss)	1.8	(0.9)	4.6	2.4
Purchases	40.4	60.1	120.4	148.3
Settlements	(31.2)	(20.6)	(75.5)	(34.9)
Sales	(3.7)	(34.0)	(32.0)	(79.3)
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	(15.2)	(15.0)	(28.6)	(15.0)
Balance at End of Period	387.4	442.0	387.4	442.0
US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	0.0	0.0	3.8	0.0
Included in Condensed Consolidated Statement of Operations	0.0	0.0	(1.2)	0.0
Included in Other Comprehensive Income (Loss)	0.0	0.0	0.0	0.0
Purchases	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	(2.6)	0.0
Sales	0.0	0.0	0.0	0.0
Transfers into Level 3	1.4	4.1	1.4	4.1
Transfers out of Level 3	0.0	0.0	0.0	0.0
Balance at End of Period	1.4	4.1	1.4	4.1
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	0.1	2.9	0.6	3.8
Included in Condensed Consolidated Statement of Operations	0.0	0.0	0.1	0.0
Included in Other Comprehensive Income (Loss)	0.0	0.4	(0.1)	0.0
Purchases	0.0	0.0	0.0	0.0
Settlements	0.0	(2.4)	(0.5)	(2.9)
Sales	0.0	0.0	0.0	0.0
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	0.0	0.0
Balance at End of Period	0.1	0.9	0.1	0.9
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	95.4	102.2	103.5	87.3
Included in Condensed Consolidated Statement of Operations	0.5	0.1	1.4	(0.2)
Included in Other Comprehensive Income (Loss)	0.0	5.2	1.7	3.0
Purchases	6.3	13.9	22.7	33.2
Settlements	(5.5)	0.0	(18.0)	0.0
Sales	(4.3)	0.0	(11.0)	(1.9)
Transfers into Level 3	5.0	0.0	5.0	0.0
Transfers out of Level 3	(3.9)	(10.1)	(11.8)	(10.1)
Balance at End of Period	93.5	111.3	93.5	111.3
Other Mortgage- and Asset-backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		3.7		3.8
Included in Condensed Consolidated Statement of Operations		0.4		0.4
Included in Other Comprehensive Income (Loss)		(0.4)		(0.4)
Purchases		0.0		0.0
Settlements		(2.9)		(3.0)
Sales		0.0		0.0
Transfers into Level 3		0.0		0.0
Transfers out of Level 3		(0.8)		(0.8)
Balance at End of Period		0.0		0.0
Preferred and Common Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	22.8	31.4	29.7	30.0
Included in Condensed Consolidated Statement of Operations	0.4	1.4	1.7	0.5
Included in Other Comprehensive Income (Loss)	2.4	1.0	0.9	3.5
Purchases	0.0	0.1	3.2	0.2
Settlements	0.0	0.0	0.0	0.0
Sales	(0.5)	(2.9)	(6.9)	(3.2)
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	(3.5)	0.0
Balance at End of Period	25.1	31.0	25.1	31.0
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	32.2	47.2	40.9	45.6
Included in Condensed Consolidated Statement of Operations	0.3	0.0	3.7	(0.7)
Included in Other Comprehensive Income (Loss)	0.4	(6.2)	(6.3)	(3.4)
Purchases	1.5	0.1	2.9	2.1
Settlements	0.0	0.0	0.0	0.0
Sales	(0.3)	(2.1)	(7.1)	(4.6)
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	0.0	0.0	0.0	0.0
Balance at End of Period	$ 34.1	$ 39.0	$ 34.1	$ 39.0