Reserve for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses	$ 2,610	$ 3,414
Excess and Surplus Lines | Prior accident years | From accident years 2016 and 2014
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses		3,200
Excess and Surplus Lines | Prior accident years | From the 2014-2016 accident years
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses	1,100
Specialty Admitted Insurance | Prior accident years | From accident years 2015, 2014 and prior accident years
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses		42
Specialty Admitted Insurance | Prior accident years | From the 2014-2016 accident years
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses	1,300
Casualty Reinsurance | Prior accident years
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses	$ 176	$ 145