Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Level 3 to level 1 Transfers, Amount	$ 3.5
Fair Value, Level 3 to level 2 Transfers, Amount	41.9
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	581.7	$ 606.8
Included in Consolidated Statement of Income	0.1	(26.2)
Included in Other Comprehensive Income	5.9	5.9
Purchases	212.7	244.6
Settlements	(147.2)	(101.8)
Sales	(65.7)	(125.7)
Transfers Into Level 3	14.5	4.1
Transfers out of Level 3	(45.4)	(26.0)
Balance at End of Year	556.6	581.7
Corporate Bonds and Notes [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	403.2	436.3
Included in Consolidated Statement of Income	(5.7)	(23.0)
Included in Other Comprehensive Income	4.9	0.9
Purchases	171.0	203.7
Settlements	(110.6)	(85.4)
Sales	(40.8)	(114.5)
Transfers Into Level 3	8.1	0.0
Transfers out of Level 3	(28.6)	(14.8)
Balance at End of Year	401.5	403.2
States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	3.8	0.0
Included in Consolidated Statement of Income	(1.2)	(0.3)
Included in Other Comprehensive Income	0.1	0.0
Purchases	0.0	0.0
Settlements	(2.6)	0.0
Sales	0.0	0.0
Transfers Into Level 3	1.4	4.1
Transfers out of Level 3	(1.5)	0.0
Balance at End of Year	0.0	3.8
Redeemable Preferred Stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	0.6	3.8
Included in Consolidated Statement of Income	0.1	0.0
Included in Other Comprehensive Income	(0.1)	0.0
Purchases	0.0	0.0
Settlements	(0.5)	(3.2)
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 Year	0.1	0.6
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	103.5	87.3
Included in Consolidated Statement of Income	1.5	0.0
Included in Other Comprehensive Income	2.4	4.3
Purchases	34.0	29.1
Settlements	(33.5)	(5.0)
Sales	(7.9)	(1.9)
Transfers Into Level 3	5.0	0.0
Transfers out of Level 3	(11.8)	(10.3)
Balance at End of Year	93.2	103.5
Other Mortgage and Asset-backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	0.0	3.8
Included in Consolidated Statement of Income	0.0	0.4
Included in Other Comprehensive Income	0.0	(0.3)
Purchases	0.0	0.0
Settlements	0.0	(3.0)
Sales	0.0	0.0
Transfers Into Level 3	0.0	0.0
Transfers out of Level 3	0.0	(0.9)
Balance at End of Year	0.0	0.0
Preferred and Common Stock [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	29.7	30.0
Included in Consolidated Statement of Income	1.4	(1.7)
Included in Other Comprehensive Income	3.4	3.4
Purchases	4.6	7.9
Settlements	0.0	(5.2)
Sales	(8.2)	(4.7)
Transfers Into Level 3	0.0	0.0
Transfers out of Level 3	(3.5)	0.0
Balance at End of Year	27.4	29.7
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	40.9	45.6
Included in Consolidated Statement of Income	4.0	(1.6)
Included in Other Comprehensive Income	(4.8)	(2.4)
Purchases	3.1	3.9
Settlements	0.0	0.0
Sales	(8.8)	(4.6)
Transfers Into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Balance at End of Year	$ 34.4	$ 40.9