Related Party Transactions - AmTrust Quota Share Arrangement (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Related Party Transaction [Line Items]
Gross premiums written	$ 852,640	$ 923,427
Net premiums earned	685,432	709,484
Net loss and loss adjustment expense (loss and LAE)	(473,324)	(480,569)
Commission expenses	(208,614)	(222,029)
Quota Share Reinsurance Agreements | AmTrust Financial Services, Inc.
Related Party Transaction [Line Items]
Gross premiums written	573,928	591,382
Net premiums earned	503,799	519,127
Net loss and loss adjustment expense (loss and LAE)	(344,529)	(353,864)
Commission expenses	$ (155,850)	$ (160,770)