Reserve for Loss and Loss Adjustment Expenses - Reconciliation of claim development to reserve for loss and LAE (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Nov. 30, 2010	Oct. 31, 2008
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	$ 3,429,637
Reinsurance Recoverables on unpaid claims	117,611
Reserve for loss and loss adjustment expenses	3,547,248	$ 2,896,496	$ 2,510,101	$ 2,271,292
Diversified Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves					$ 98,827	$ 755,554
Operating Segments | Diversified Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	1,142,743
Reinsurance Recoverables on unpaid claims	44,628
Reserve for loss and loss adjustment expenses	1,187,371
Operating Segments | AmTrust Reinsurance segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	2,268,150
Reinsurance Recoverables on unpaid claims	22,831
Reserve for loss and loss adjustment expenses	2,290,981
Other
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	18,744
Reinsurance Recoverables on unpaid claims	50,152
Reserve for loss and loss adjustment expenses	68,896
Property | Diversified Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	79,354
Reinsurance Recoverables on unpaid claims	2,101
Reserve for loss and loss adjustment expenses	81,455
Casualty Personal Auto - Pro-rata | Diversified Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	83,466
Reinsurance Recoverables on unpaid claims	13,438
Reserve for loss and loss adjustment expenses	96,904
Casualty Commercial Auto - Pro-rata | Diversified Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	125,094
Reinsurance Recoverables on unpaid claims	1,592
Reserve for loss and loss adjustment expenses	126,686
Property XOL | Diversified Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	67,887
Reinsurance Recoverables on unpaid claims	0
Reserve for loss and loss adjustment expenses	67,887
Commercial Auto XOL | Diversified Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	198,621
Reinsurance Recoverables on unpaid claims	0
Reserve for loss and loss adjustment expenses	198,621
Workers' Compensation XOL | Diversified Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	282,983
Reinsurance Recoverables on unpaid claims	0
Reserve for loss and loss adjustment expenses	282,983
Other Liability - XOL | Diversified Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	77,988
Reinsurance Recoverables on unpaid claims	25,446
Reserve for loss and loss adjustment expenses	103,434
Accident and Health | Diversified Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	61,108
Reinsurance Recoverables on unpaid claims	0
Reserve for loss and loss adjustment expenses	61,108
International | Diversified Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	66,391
Reinsurance Recoverables on unpaid claims	2,051
Reserve for loss and loss adjustment expenses	68,442
Workers' Compensation | AmTrust Reinsurance segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	1,174,164
Reinsurance Recoverables on unpaid claims	0
Reserve for loss and loss adjustment expenses	1,174,164
General Liability | AmTrust Reinsurance segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	362,742
Reinsurance Recoverables on unpaid claims	0
Reserve for loss and loss adjustment expenses	362,742
Commercial Auto Liability | AmTrust Reinsurance segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	276,248
Reinsurance Recoverables on unpaid claims	16,991
Reserve for loss and loss adjustment expenses	293,239
European Hospital Liability | AmTrust Reinsurance segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	227,384
Reinsurance Recoverables on unpaid claims	0
Reserve for loss and loss adjustment expenses	227,384
All other lines | AmTrust Reinsurance segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	72,564
Reinsurance Recoverables on unpaid claims	0
Reserve for loss and loss adjustment expenses	72,564
Total | Diversified Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	1,042,892
Reinsurance Recoverables on unpaid claims	44,628
Reserve for loss and loss adjustment expenses	1,087,520
Total | AmTrust Reinsurance segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	2,113,102
Reinsurance Recoverables on unpaid claims	16,991
Reserve for loss and loss adjustment expenses	2,130,093
Other | Diversified Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	99,851
Reinsurance Recoverables on unpaid claims	0
Reserve for loss and loss adjustment expenses	99,851
Other | AmTrust Reinsurance segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Net Reserves	155,048
Reinsurance Recoverables on unpaid claims	5,840
Reserve for loss and loss adjustment expenses	$ 160,888