Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	$ 551.3	$ 594.4	$ 581.7	$ 606.8
Included in Condensed Consolidated Statement of Income	5.1	(6.2)	0.6	(10.7)
Included in Other Comprehensive Income (Loss)	(1.3)	11.5	(3.8)	6.0
Purchases	60.9	67.8	101.0	109.6
Settlements	(37.1)	(8.7)	(59.9)	(14.9)
Sales	(18.3)	(12.0)	(48.2)	(50.0)
Transfers into Level 3	0.0	0.0	0.0	0.0
Transfers out of Level 3	(14.0)	0.0	(24.8)	0.0
Balance at End of Period	546.6	646.8	546.6	646.8
Fair Value, Level 3 to level 1 Transfers, Amount	(3.5)		(3.5)
Fair Value, Level 3 to level 2 Transfers, Amount	(10.5)		(21.3)
Corporate Bonds and Notes [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	391.2	431.8	403.2	436.3
Included in Condensed Consolidated Statement of Income	(0.6)	(6.3)	(3.9)	(8.8)
Included in Other Comprehensive Income (Loss)	1.4	4.1	2.8	3.3
Purchases	50.2	48.1	80.0	88.2
Settlements	(29.3)	(8.5)	(44.3)	(14.3)
Sales	(6.3)	(9.8)	(28.3)	(45.3)
Transfers into Level 3	0.0		0.0
Transfers out of Level 3	(10.5)		(13.4)
Balance at End of Period	396.1	459.4	396.1	459.4
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	2.6		3.8
Included in Condensed Consolidated Statement of Income	0.0		(1.2)
Included in Other Comprehensive Income (Loss)	0.0		0.0
Purchases	0.0		0.0
Settlements	(2.6)		(2.6)
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	0.0		0.0
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	0.2	3.3	0.6	3.8
Included in Condensed Consolidated Statement of Income	0.1	0.0	0.1	0.0
Included in Other Comprehensive Income (Loss)	(0.2)	(0.3)	(0.1)	(0.4)
Purchases	0.0	0.0	0.0	0.0
Settlements	0.0	(0.1)	(0.5)	(0.5)
Sales	0.0	0.0	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	0.1	2.9	0.1	2.9
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	95.8	83.2	103.5	87.3
Included in Condensed Consolidated Statement of Income	0.7	0.3	0.9	(0.3)
Included in Other Comprehensive Income (Loss)	(0.2)	3.0	1.7	(2.2)
Purchases	7.2	17.6	16.4	19.3
Settlements	(5.2)	0.0	(12.5)	0.0
Sales	(2.9)	(1.9)	(6.7)	(1.9)
Transfers into Level 3	0.0		0.0
Transfers out of Level 3	0.0		(7.9)
Balance at End of Period	95.4	102.2	95.4	102.2
Other Mortgage- and Asset-backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period		3.6		3.8
Included in Condensed Consolidated Statement of Income		0.0		0.0
Included in Other Comprehensive Income (Loss)		0.2		0.0
Purchases		0.0		0.0
Settlements		(0.1)		(0.1)
Sales		0.0		0.0
Balance at End of Period		3.7		3.7
Preferred and Common Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	29.1	30.3	29.7	30.0
Included in Condensed Consolidated Statement of Income	1.5	0.1	1.3	(0.9)
Included in Other Comprehensive Income (Loss)	(2.2)	1.2	(1.5)	2.5
Purchases	2.9	0.1	3.2	0.1
Settlements	0.0	0.0	0.0	0.0
Sales	(5.0)	(0.3)	(6.4)	(0.3)
Transfers into Level 3	0.0		0.0
Transfers out of Level 3	(3.5)		(3.5)
Balance at End of Period	22.8	31.4	22.8	31.4
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Period	32.4	42.2	40.9	45.6
Included in Condensed Consolidated Statement of Income	3.4	(0.3)	3.4	(0.7)
Included in Other Comprehensive Income (Loss)	(0.1)	3.3	(6.7)	2.8
Purchases	0.6	2.0	1.4	2.0
Settlements	0.0	0.0	0.0	0.0
Sales	(4.1)	0.0	(6.8)	(2.5)
Transfers into Level 3	0.0		0.0
Transfers out of Level 3	0.0		0.0
Balance at End of Period	$ 32.2	$ 47.2	$ 32.2	$ 47.2