Schedule 1 - Investments Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2017 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	$ 6,394.0
Amount Carried in Balance Sheet	6,804.9
Fixed Maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	5,021.6
Fair Value	5,382.7
Amount Carried in Balance Sheet	5,382.7
US Government and Government Agencies and Authorities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	542.7
Fair Value	556.1
Amount Carried in Balance Sheet	556.1
States and Political Subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	1,595.5
Fair Value	1,701.8
Amount Carried in Balance Sheet	1,701.8
Foreign Government Debt [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	3.0
Fair Value	3.2
Amount Carried in Balance Sheet	3.2
Other Corporate Bonds and Notes [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	2,745.8
Fair Value	2,980.6
Amount Carried in Balance Sheet	2,980.6
Redeemable Preferred Stocks [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	0.1
Fair Value	0.1
Amount Carried in Balance Sheet	0.1
Collateralized Loan Obligations [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	134.1
Fair Value	139.8
Amount Carried in Balance Sheet	139.8
Other Mortgage and Asset-backed Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	0.4
Fair Value	1.1
Amount Carried in Balance Sheet	1.1
Equity Securities, Investment Summary [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	476.2
Fair Value	526.0
Amount Carried in Balance Sheet	526.0
Preferred Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	68.2
Fair Value	78.8
Amount Carried in Balance Sheet	78.8
Common Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	15.5
Fair Value	24.8
Amount Carried in Balance Sheet	24.8
Other Equity Interests [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	392.5
Fair Value	422.4
Amount Carried in Balance Sheet	422.4
Fair Value Option Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	77.5
Fair Value	77.5
Amount Carried in Balance Sheet	77.5
Equity Method Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	161.0
Amount Carried in Balance Sheet	161.0
Loans, Real Estate and Other Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	422.2
Amount Carried in Balance Sheet	422.2
Short-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	235.5
Amount Carried in Balance Sheet	$ 235.5