Other Reinsurance (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) employees director	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Effects of Reinsurance [Line Items]
Ceded earned premiums	$ 27.4	$ 31.6	$ 18.2
Assumed earned premiums	92.3	85.2	72.9
Catastrophe Reinsurance
Effects of Reinsurance [Line Items]
Ceded earned premiums	20.5	20.5	11.0
Non-Catastrophe Reinsurance
Effects of Reinsurance [Line Items]
Assumed earned premiums	$ 92.3	85.2	72.9
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	$ 19.4	20.0	20.7
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	$ 5.2	$ 5.6	$ 5.9
Old Reliable Casualty Company | Trinity
Effects of Reinsurance [Line Items]
Percentage of written business assumed	100.00%