Other Reinsurance (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) employees director	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Effects of Reinsurance [Line Items]
Ceded earned premiums	$ 31.2	$ 27.4	$ 31.6
Assumed earned premiums	50.8	92.3	85.2
Catastrophe Reinsurance
Effects of Reinsurance [Line Items]
Ceded earned premiums	26.7	20.5	20.5
Non-Catastrophe Reinsurance
Effects of Reinsurance [Line Items]
Assumed earned premiums	$ 73.8	92.3	85.2
Capitol County Mutual Fire Insurance Company
Effects of Reinsurance [Line Items]
Number of employee board members | employees	5
Number of board of director members | director	5
Old Reliable Casualty Company
Effects of Reinsurance [Line Items]
Number of employee board members | employees	9
Number of board of director members | director	9
Capitol County Mutual Fire Insurance Company
Effects of Reinsurance [Line Items]
Assumed earned premiums	$ 18.1	19.4	20.0
Capitol County Mutual Fire Insurance Company | Trinity
Effects of Reinsurance [Line Items]
Percentage of written business assumed	100.00%
Old Reliable Casualty Company
Effects of Reinsurance [Line Items]
Assumed earned premiums	$ 4.9	$ 5.2	$ 5.6
Old Reliable Casualty Company | Trinity
Effects of Reinsurance [Line Items]
Percentage of written business assumed	100.00%