Schedule 1 - Investments Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2016 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	$ 6,282.1
Amount Carried in Balance Sheet	6,607.5
Fixed Maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	4,846.8
Fair Value	5,124.9
Amount Carried in Balance Sheet	5,124.9
US Government and Government Agencies and Authorities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	321.2
Fair Value	336.3
Amount Carried in Balance Sheet	336.3
States and Political Subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	1,640.6
Fair Value	1,714.9
Amount Carried in Balance Sheet	1,714.9
Foreign Government Debt [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	3.5
Fair Value	3.4
Amount Carried in Balance Sheet	3.4
Other Corporate Bonds and Notes [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	2,758.9
Fair Value	2,944.8
Amount Carried in Balance Sheet	2,944.8
Redeemable Preferred Stocks [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	0.5
Fair Value	0.6
Amount Carried in Balance Sheet	0.6
Collateralized Loan Obligations [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	121.2
Fair Value	122.8
Amount Carried in Balance Sheet	122.8
Other Mortgage and Asset-backed Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	0.9
Fair Value	2.1
Amount Carried in Balance Sheet	2.1
Equity Securities, Investment Summary [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	434.4
Fair Value	481.7
Amount Carried in Balance Sheet	481.7
Preferred Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	76.6
Fair Value	82.5
Amount Carried in Balance Sheet	82.5
Common Stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	38.4
Fair Value	45.2
Amount Carried in Balance Sheet	45.2
Other Equity Interests [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	319.4
Fair Value	354.0
Amount Carried in Balance Sheet	354.0
Fair Value Option Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	111.4
Fair Value	111.4
Amount Carried in Balance Sheet	111.4
Equity Method Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	175.9
Amount Carried in Balance Sheet	175.9
Loans, Real Estate and Other Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	439.9
Amount Carried in Balance Sheet	439.9
Short-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	273.7
Amount Carried in Balance Sheet	$ 273.7