Reserve for Losses and Loss Adjustment Expenses - Reserves for Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	$ 1,377,461	$ 1,194,088	$ 989,825	$ 761,128
Reinsurance recoverable on unpaid losses	668,045	467,371	302,524
Reserve for losses and loss adjustment expenses gross of reinsurance recoverables on unpaid losses and loss adjustment expenses at end of period	2,045,506	$ 1,661,459	$ 1,292,349
Casualty Reinsurance
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	283,676
Fronting and Programs | Specialty Admitted Insurance
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	48,727
Excluding Commercial Auto | Excess and Surplus Lines
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	568,814
Commercial Auto | Excess and Surplus Lines
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	433,053
Individual Risk Workers' Compensation | Specialty Admitted Insurance
Claims Development [Line Items]
Net reserve for losses and loss adjustment expenses	$ 43,191