Investments - Amortized Cost and Estimated Fair Value of investments (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,372.7	$ 6,284.5
Gross Unrealized Gains	570.2	226.3
Gross Unrealized Losses	(20.8)	(86.6)
Fair Value	6,922.1	6,424.2
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	784.7	865.9
Gross Unrealized Gains	32.5	14.8
Gross Unrealized Losses	(1.3)	(15.0)
Fair Value	815.9	865.7
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,386.4	1,553.7
Gross Unrealized Gains	130.5	74.0
Gross Unrealized Losses	(1.1)	(8.6)
Fair Value	1,515.8	1,619.1
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17.2	6.5
Gross Unrealized Gains	1.2	0.0
Gross Unrealized Losses	(1.6)	(0.6)
Fair Value	16.8	5.9
Corporate Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,465.0	3,307.8
Gross Unrealized Gains	401.8	135.1
Gross Unrealized Losses	(7.1)	(49.1)
Fair Value	3,859.7	3,393.8
Redeemable Preferred Stocks
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6.8
Gross Unrealized Gains	0.0
Gross Unrealized Losses	(0.1)
Fair Value	6.7
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	624.6	535.7
Gross Unrealized Gains	2.1	1.5
Gross Unrealized Losses	(8.5)	(13.2)
Fair Value	618.2	524.0
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	88.0	14.9
Gross Unrealized Gains	2.1	0.9
Gross Unrealized Losses	(1.1)	(0.1)
Fair Value	$ 89.0	$ 15.7