Schedule 1 - Investments Other Than Investments in Related Parties (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	$ 7,929.8
Amount Carried in Balance Sheet	8,069.5
Convertible Debt, Current	31.5
Equity Securities, FV-NI	(64.3)	$ 0.0	$ 0.0
Fixed Maturities [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	6,284.5
Fair Value	6,424.2
Amount Carried in Balance Sheet	6,424.2
US Government and Government Agencies and Authorities [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	865.9
Fair Value	865.7
Amount Carried in Balance Sheet	865.7
States and Political Subdivisions [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	1,553.7
Fair Value	1,619.1
Amount Carried in Balance Sheet	1,619.1
Foreign Government Debt [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	6.5
Fair Value	5.9
Amount Carried in Balance Sheet	5.9
Other Corporate Bonds and Notes [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	3,307.8
Fair Value	3,393.8
Amount Carried in Balance Sheet	3,393.8
Collateralized Loan Obligations [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	535.7
Fair Value	524.0
Amount Carried in Balance Sheet	524.0
Other Mortgage and Asset-backed Securities [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	14.9
Fair Value	15.7
Amount Carried in Balance Sheet	15.7
Equity Securities, Investment Summary [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	684.4
Fair Value	684.4
Amount Carried in Balance Sheet	684.4
Preferred Stock [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	54.2
Fair Value	54.2
Amount Carried in Balance Sheet	54.2
Common Stock [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	10.9
Fair Value	10.9
Amount Carried in Balance Sheet	10.9
Other Equity Interests [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	619.3
Fair Value	619.3
Amount Carried in Balance Sheet	619.3
Equity Method Investments [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	187.0
Amount Carried in Balance Sheet	187.0
Loans, Real Estate and Other Investments [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	414.8
Amount Carried in Balance Sheet	414.8
Short-term Investments [Member]
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost	286.1
Amount Carried in Balance Sheet	$ 286.1