Reserve for Loss and Loss Adjustment Expenses - Prior year development (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net incurred losses related to Prior years				$ 448,637	$ 283,573	$ 131,077
Percentage of prior year net loss and loss adjustment expense reserves				19.00%	15.70%	8.70%
Adverse development, net				$ 403,211	$ 247,175	$ 76,969
Diversified Reinsurance segment
Segment Reporting Information [Line Items]
Adverse development, net				2,326	5,441	11,082
AmTrust Reinsurance segment
Segment Reporting Information [Line Items]
Adverse development, net	$ 151,874	$ (210,433)	$ 139,024	399,200	239,896	54,000
Increase in incurred losses associated with prior year earned premium				45,426	37,212
Premium earned attributable to to prior accident years				75,359	57,026
AmTrust Reinsurance segment | Commercial Auto Liability
Segment Reporting Information [Line Items]
Adverse development, net				76,207	(19,877)
AmTrust Reinsurance segment | Workers' Compensation
Segment Reporting Information [Line Items]
Adverse development, net				151,269	126,603
AmTrust Reinsurance segment | General Liability
Segment Reporting Information [Line Items]
Adverse development, net				78,317	(90,784)
Other
Segment Reporting Information [Line Items]
Adverse development, net				$ 1,685	$ 1,838	$ 11,887