Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
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	$ 9.5	(41.9)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	556.6	581.7
Included in Consolidated Statement of Income	7.0	0.1
Included in Other Comprehensive Income	(18.4)	5.9
Purchases	663.3	212.7
Settlements	(166.6)	(147.2)
Sales	(109.5)	(65.7)
Transfers Into Level 3	36.3	14.5
Transfers out of Level 3	(71.3)	(45.4)
Balance at End of Year	897.4	556.6
Corporate Bonds and Notes [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	401.5	403.2
Included in Consolidated Statement of Income	4.4	(5.7)
Included in Other Comprehensive Income	(2.2)	4.9
Purchases	201.8	171.0
Settlements	(108.1)	(110.6)
Sales	(109.5)	(40.8)
Transfers Into Level 3	2.4	8.1
Transfers out of Level 3	(7.7)	(28.6)
Balance at End of Year	382.6	401.5
States and Political Subdivisions [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	(1.2)
Included in Other Comprehensive Income	0.0	0.1
Purchases	1.8	0.0
Settlements	0.0	(2.6)
Sales	0.0	0.0
Transfers Into Level 3	0.0	1.4
Transfers out of Level 3	(1.8)	(1.5)
Balance at End of Year	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 Year	0.1	0.6
Included in Consolidated Statement of Income	(0.1)	0.1
Included in Other Comprehensive Income	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	0.0
Transfers out of Level 3	0.0	0.0
Balance at End of Year	0.0	0.1
Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	93.2	103.5
Included in Consolidated Statement of Income	2.7	1.5
Included in Other Comprehensive Income	(16.3)	2.4
Purchases	449.7	34.0
Settlements	(58.3)	(33.5)
Sales	0.0	7.9
Transfers Into Level 3	33.9	5.0
Transfers out of Level 3	0.0	11.8
Balance at End of Year	504.9	93.2
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	0.0
Included in Consolidated Statement of Income	0.0	0.0
Included in Other Comprehensive Income	0.1	0.0
Purchases	10.0	0.0
Settlements	0.2	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 Year	9.9	0.0
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Transfers out of Level 3	61.8
Equity Securities, Investment Summary [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	27.4	29.7
Included in Consolidated Statement of Income	0.0	1.4
Included in Other Comprehensive Income	0.0	3.4
Purchases	0.0	4.6
Settlements	0.0	0.0
Sales	0.0	8.2
Transfers Into Level 3	0.0	0.0
Transfers out of Level 3	27.4	3.5
Balance at End of Year	0.0	27.4
Other Equity Interests [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	34.4	40.9
Included in Consolidated Statement of Income	0.0	4.0
Included in Other Comprehensive Income	0.0	(4.8)
Purchases	0.0	3.1
Settlements	0.0	0.0
Sales	0.0	(8.8)
Transfers Into Level 3	0.0	0.0
Transfers out of Level 3	(34.4)	0.0
Balance at End of Year	$ 0.0	$ 34.4