Reserve for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses	$ (69,008)	$ (17,672)	$ (21,472)
Excess and Surplus Lines
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses	(51,200)	(15,000)	(20,000)
Specialty Admitted Insurance | Accident Years Prior to 2018
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses	5,300
Specialty Admitted Insurance | Accident years 2014 through 2016
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses		5,600
Specialty Admitted Insurance | Accident years 2010 through 2015
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses			2,700
Casualty Reinsurance | Accident Years 2011 Through 2016
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses	(23,100)
Casualty Reinsurance | Accident years at least four years old
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses		(8,200)
Casualty Reinsurance | Accident years 2010 through 2013
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses			(4,200)
Excluding Commercial Auto | Excess and Surplus Lines
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses	6,200
Excluding Commercial Auto | Excess and Surplus Lines | Accident year 2017
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses		5,700
Excluding Commercial Auto | Excess and Surplus Lines | Accident years 2014 through 2016
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses			18,600
Commercial Auto | Excess and Surplus Lines | Accident years 2016 and 2017
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses	$ (57,400)
Commercial Auto | Excess and Surplus Lines | Accident year 2016
Favorable development on prior year loss reserves
Reserves for losses and loss adjustment expenses		$ (20,700)	$ (38,700)