Reserve for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses	$ (2,224)	$ (1,715)	$ 386	$ 1,699
Excess and Surplus Lines
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses	58	1,400	1,200	4,700
Excess and Surplus Lines | 2017 Catastrophe Losses
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses	1,900		1,900
Specialty Admitted Insurance
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses	167	(949)	1,500	(907)
Casualty Reinsurance
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses	$ (2,400)	$ (2,200)	$ (2,300)	$ (2,100)