SCHEDULE I - SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES - Summary (Details) - Available-for-sale: - Fixed maturities: $ in Millions	Dec. 31, 2018 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	$ 2,188.2
Fair value	2,159.5
Amount at which shown in the balance sheet	2,159.5
U.S. treasury securities and obligations of U.S. government agencies
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	438.4
Fair value	432.4
Amount at which shown in the balance sheet	432.4
Obligations of states and political subdivisions
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	408.2
Fair value	414.6
Amount at which shown in the balance sheet	414.6
Corporate securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	551.7
Fair value	540.6
Amount at which shown in the balance sheet	540.6
U.S. government agencies mortgage-backed securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost or amortized cost	789.9
Fair value	771.9
Amount at which shown in the balance sheet	$ 771.9