Reserve for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses	$ 7,564	$ 5,333	$ 9,263	$ 14,748
Excess and Surplus Lines | Prior accident years | From the 2015 and 2014 accident years
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses	5,100
Excess and Surplus Lines | Prior accident years | From the 2013, 2014, and 2015 accident years
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses		5,800		13,800
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			9,800
Specialty Admitted Insurance | Prior accident years | From the 2015 and 2014 accident years
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses	3,000
Specialty Admitted Insurance | Prior accident years | From the 2013, 2014, and 2015 accident years
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses		1,600
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			2,100
Specialty Admitted Insurance | Prior accident years | From the 2010-2014 accident years
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses				2,500
Casualty Reinsurance | Prior accident years | From the 2013 and 2011 accident years
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses	$ (581)
Casualty Reinsurance | Prior accident years | From the 2013 and 2012 accident years
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses		$ (2,000)		$ (1,500)
Casualty Reinsurance | Prior accident years | From the 2013-2011 accident years
Favorable development on prior year loss reserves
Reserve for losses and loss adjustment expenses			$ (2,600)