Segment Information - Net Premiums (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 645,589	$ 684,072	$ 1,494,922	$ 1,584,620
% of Total Net premiums written	100.00%	100.00%	100.00%	100.00%
Net premiums earned	$ 667,346	$ 711,125	$ 1,352,778	$ 1,420,609
% of Total Net premiums earned	100.00%	100.00%	100.00%	100.00%
Operating segments | Diversified Reinsurance
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 154,278	$ 137,247	$ 429,231	$ 464,743
% of Total Net premiums written	23.90%	20.10%	28.70%	29.30%
Net premiums earned	$ 191,497	$ 204,219	$ 385,631	$ 406,061
% of Total Net premiums earned	28.70%	28.70%	28.50%	28.60%
Operating segments | Diversified Reinsurance | Property
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 20,464	$ 21,059	$ 81,445	$ 94,436
% of Total Net premiums written	3.20%	3.10%	5.40%	6.00%
Net premiums earned	$ 34,028	$ 39,632	$ 72,410	$ 79,526
% of Total Net premiums earned	5.10%	5.50%	5.40%	5.60%
Operating segments | Diversified Reinsurance | Casualty
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 90,418	$ 84,219	$ 213,358	$ 261,777
% of Total Net premiums written	14.00%	12.30%	14.30%	16.50%
Net premiums earned	$ 107,588	$ 121,563	$ 216,234	$ 244,713
% of Total Net premiums earned	16.10%	17.10%	15.90%	17.20%
Operating segments | Diversified Reinsurance | Accident and Health
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 13,678	$ 8,909	$ 56,480	$ 57,558
% of Total Net premiums written	2.10%	1.30%	3.80%	3.60%
Net premiums earned	$ 21,342	$ 20,409	$ 42,974	$ 41,098
% of Total Net premiums earned	3.20%	2.90%	3.20%	2.90%
Operating segments | Diversified Reinsurance | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 29,718	$ 23,060	$ 77,948	$ 50,972
% of Total Net premiums written	4.60%	3.40%	5.20%	3.20%
Net premiums earned	$ 28,539	$ 22,615	$ 54,013	$ 40,724
% of Total Net premiums earned	4.30%	3.20%	4.00%	2.90%
Operating segments | AmTrust Reinsurance
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 491,311	$ 546,735	$ 1,065,691	$ 1,119,787
% of Total Net premiums written	76.10%	79.90%	71.30%	70.70%
Net premiums earned	$ 475,849	$ 506,816	$ 967,147	$ 1,014,458
% of Total Net premiums earned	71.30%	71.30%	71.50%	71.40%
Operating segments | AmTrust Reinsurance | Small Commercial Business
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 279,486	$ 340,840	$ 647,240	$ 733,406
% of Total Net premiums written	43.30%	49.80%	43.30%	46.30%
Net premiums earned	$ 293,514	$ 315,100	$ 609,223	$ 632,009
% of Total Net premiums earned	44.00%	44.30%	45.00%	44.50%
Operating segments | AmTrust Reinsurance | Specialty Program
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 103,196	$ 100,082	$ 192,327	$ 191,951
% of Total Net premiums written	16.00%	14.60%	12.90%	12.10%
Net premiums earned	$ 96,739	$ 92,262	$ 185,233	$ 192,010
% of Total Net premiums earned	14.50%	13.00%	13.70%	13.50%
Operating segments | AmTrust Reinsurance | Specialty Risk and Extended Warranty
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 108,629	$ 105,813	$ 226,124	$ 194,430
% of Total Net premiums written	16.80%	15.50%	15.10%	12.30%
Net premiums earned	$ 85,596	$ 99,454	$ 172,691	$ 190,439
% of Total Net premiums earned	12.80%	14.00%	12.80%	13.40%
Other
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 0	$ 90	$ 0	$ 90
% of Total Net premiums written	0.00%	0.00%	0.00%	0.00%
Net premiums earned	$ 0	$ 90	$ 0	$ 90
% of Total Net premiums earned	0.00%	0.00%	0.00%	0.00%