Other Reinsurance Assumed Insurance (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) employees director	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Assumed Insurance [Line Items]
Assumed Premiums Earned	$ 72.9	$ 67.3	$ 60.1
Capitol [Member]
Assumed Insurance [Line Items]
Number of Company Employee Board Members | employees	5
Number of Board of Director Members | director	5
ORCC [Member]
Assumed Insurance [Line Items]
Number of Company Employee Board Members | employees	9
Number of Board of Director Members | director	9
Capitol [Member]
Assumed Insurance [Line Items]
Assumed Premiums Earned	$ 20.7	21.3	21.8
Capitol [Member] | Trinity [Member]
Assumed Insurance [Line Items]
Earned Premiums Assumed, Percentage	100.00%
ORCC [Member]
Assumed Insurance [Line Items]
Assumed Premiums Earned	$ 5.9	6.2	6.5
ORCC [Member] | Trinity [Member]
Assumed Insurance [Line Items]
Earned Premiums Assumed, Percentage	100.00%
Non-Catastrophe Reinsurance [Member]
Assumed Insurance [Line Items]
Assumed Premiums Earned	$ 72.9	$ 67.3	$ 60.0