Segment Information - Reconciliation of Assets from Reportable Segments to Consolidated (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	$ 3,054,704	$ 3,568,196
Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	2,570,192	3,011,647
Operating Segments | Diversified Reinsurance
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	156,237	167,845
Operating Segments | AmTrust Reinsurance
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	2,413,955	2,843,802
Corporate, Non-Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	484,512	556,549
Corporate, Non-Segment | Diversified Reinsurance
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	0	0
Corporate, Non-Segment | AmTrust Reinsurance
Segment Reporting, Asset Reconciling Item [Line Items]
Assets	$ 0	$ 0