Segment Information - Net Premiums (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 684,072	$ 650,427	$ 1,584,620	$ 1,443,258
% of Total Net premiums written	100.00%	100.00%	100.00%	100.00%
Net premiums earned	$ 711,125	$ 637,564	$ 1,420,609	$ 1,253,573
% of Total Net premiums earned	100.00%	100.00%	100.00%	100.00%
Operating segments [Member] | Diversified Reinsurance segment [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 137,247	$ 161,294	$ 464,743	$ 447,430
% of Total Net premiums written	20.10%	24.80%	29.30%	31.00%
Net premiums earned	$ 204,219	$ 190,755	$ 406,061	$ 363,011
% of Total Net premiums earned	28.70%	29.90%	28.60%	29.00%
Operating segments [Member] | AmTrust Reinsurance segment [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 546,735	$ 489,133	$ 1,119,787	$ 995,828
% of Total Net premiums written	79.90%	75.20%	70.70%	69.00%
Net premiums earned	$ 506,816	$ 446,809	$ 1,014,458	$ 890,562
% of Total Net premiums earned	71.30%	70.10%	71.40%	71.00%
Operating segments [Member] | Property [Member] | Diversified Reinsurance segment [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 21,059	$ 31,516	$ 94,436	$ 93,385
% of Total Net premiums written	3.10%	4.80%	6.00%	6.50%
Net premiums earned	$ 39,632	$ 40,392	$ 79,526	$ 73,102
% of Total Net premiums earned	5.50%	6.30%	5.60%	5.80%
Operating segments [Member] | Casualty [Member] | Diversified Reinsurance segment [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 84,219	$ 94,692	$ 261,777	$ 249,972
% of Total Net premiums written	12.30%	14.60%	16.50%	17.30%
Net premiums earned	$ 121,563	$ 107,677	$ 244,713	$ 207,843
% of Total Net premiums earned	17.10%	16.90%	17.20%	16.60%
Operating segments [Member] | Accident and Health [Member] | Diversified Reinsurance segment [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 8,909	$ 13,723	$ 57,558	$ 53,295
% of Total Net premiums written	1.30%	2.10%	3.60%	3.70%
Net premiums earned	$ 20,409	$ 19,548	$ 41,098	$ 37,352
% of Total Net premiums earned	2.90%	3.10%	2.90%	3.00%
Operating segments [Member] | International [Member] | Diversified Reinsurance segment [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 23,060	$ 21,363	$ 50,972	$ 50,778
% of Total Net premiums written	3.40%	3.30%	3.20%	3.50%
Net premiums earned	$ 22,615	$ 23,138	$ 40,724	$ 44,714
% of Total Net premiums earned	3.20%	3.60%	2.90%	3.60%
Operating segments [Member] | Small Commercial Business [Member] | AmTrust Reinsurance segment [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 340,840	$ 306,425	$ 733,406	$ 668,924
% of Total Net premiums written	49.80%	47.10%	46.30%	46.40%
Net premiums earned	$ 315,100	$ 279,612	$ 632,009	$ 544,103
% of Total Net premiums earned	44.30%	43.90%	44.50%	43.40%
Operating segments [Member] | Specialty Program [Member] | AmTrust Reinsurance segment [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 100,082	$ 92,507	$ 191,951	$ 169,298
% of Total Net premiums written	14.60%	14.20%	12.10%	11.70%
Net premiums earned	$ 92,262	$ 77,488	$ 192,010	$ 161,687
% of Total Net premiums earned	13.00%	12.20%	13.50%	12.90%
Operating segments [Member] | Specialty Risk and Extended Warranty [Member] | AmTrust Reinsurance segment [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 105,813	$ 90,201	$ 194,430	$ 157,606
% of Total Net premiums written	15.50%	13.90%	12.30%	10.90%
Net premiums earned	$ 99,454	$ 89,709	$ 190,439	$ 184,772
% of Total Net premiums earned	14.00%	14.00%	13.40%	14.70%
Other [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Net premiums written	$ 90	$ 0	$ 90	$ 0
% of Total Net premiums written	0.00%	0.00%	0.00%	0.00%
Net premiums earned	$ 90	$ 0	$ 90	$ 0
% of Total Net premiums earned	0.00%	0.00%	0.00%	0.00%