Losses and Loss Adjustment Expenses - Reconciliation of Incurred and Paid Loss Development (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Atrium
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	$ 197,387
Reinsurance recoverable on unpaid losses	28,297
Gross liability for unpaid losses and LAE before unallocated loss adjustment expenses and fair value adjustments	225,684	$ 235,406
StarStone
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liability for unpaid losses and LAE before unallocated loss adjustment expenses and fair value adjustments	1,851,147	1,584,088
StarStone | General casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	578,272
Reinsurance recoverable on unpaid losses	94,518
Gross liability for unpaid losses and LAE before unallocated loss adjustment expenses and fair value adjustments	672,790	508,864
StarStone | Marine
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	300,659
Reinsurance recoverable on unpaid losses	67,728
Gross liability for unpaid losses and LAE before unallocated loss adjustment expenses and fair value adjustments	368,387	367,537
StarStone | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	250,811
Reinsurance recoverable on unpaid losses	268,961
Gross liability for unpaid losses and LAE before unallocated loss adjustment expenses and fair value adjustments	519,772	440,613
StarStone | Aerospace
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	70,658
Reinsurance recoverable on unpaid losses	41,591
Gross liability for unpaid losses and LAE before unallocated loss adjustment expenses and fair value adjustments	112,249	107,619
StarStone | Workers' compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and allocated LAE, net of reinsurance	111,334
Reinsurance recoverable on unpaid losses	66,615
Gross liability for unpaid losses and LAE before unallocated loss adjustment expenses and fair value adjustments	$ 177,949	$ 159,455