Segment Information - Net Income (Loss) and Total Assets (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Segment	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of reporting segments | Segment			2
Net Income (Loss) [Abstract]
Gross premiums written	$ 630,972	$ 706,854	$ 2,259,597	$ 2,259,290
Net premiums written	617,330	690,653	2,201,950	2,133,911
Net premiums earned	653,866	698,278	2,074,475	1,951,851
Other insurance revenue	2,488	2,345	7,816	8,696
Net loss and loss adjustment expense (loss and LAE)	(535,968)	(466,751)	(1,545,157)	(1,297,361)
Commission and other acquisition expenses	(193,462)	(206,706)	(625,530)	(587,501)
General and administrative expenses	(9,366)	(9,797)	(28,059)	(29,025)
Underwriting (loss) income	(82,442)	17,369	(116,455)	46,660
Reconciliation to net income
Net investment income and realized gains on investment	46,682	37,566	131,808	111,802
Interest and amortization expenses	(4,829)	(6,856)	(18,430)	(21,314)
Accelerated amortization of senior note issuance cost	0	0	(2,809)	(2,345)
Amortization of intangible assets	(533)	(616)	(1,599)	(1,846)
Foreign exchange gains (losses)	(3,550)	687	(12,193)	6,474
Other general and administrative expenses	(10,126)	(7,155)	(24,193)	(20,713)
Income tax expense	(256)	(199)	(1,017)	(1,206)
Net (loss) income	$ (55,054)	$ 40,796	$ (44,888)	$ 117,512
Net loss and LAE ratio	81.60%	66.60%	74.20%	66.20%
Commission and other acquisition expense ratio	29.50%	29.50%	30.00%	30.00%
General and administrative expense ratio	3.00%	2.40%	2.50%	2.50%
Expense ratio	32.50%	31.90%	32.50%	32.50%
Combined ratio	114.10%	98.50%	106.70%	98.70%
Assets [Abstract]
Total assets	$ 6,839,097		$ 6,839,097		$ 6,252,299
AmTrust Reinsurance segment [Member]
Net Income (Loss) [Abstract]
Gross premiums written	420,019	$ 520,104	1,575,677	$ 1,591,902
Operating segments [Member]
Assets [Abstract]
Total assets - reportable segments	6,260,044		6,260,044		5,687,387
Corporate assets	579,053		579,053		564,912
Total assets	6,839,097		6,839,097		6,252,299
Operating segments [Member] | Diversified Reinsurance segment [Member]
Net Income (Loss) [Abstract]
Gross premiums written	210,953	186,750	683,839	667,388
Net premiums written	207,137	179,092	671,880	626,522
Net premiums earned	217,513	175,141	623,574	538,152
Other insurance revenue	2,488	2,345	7,816	8,696
Net loss and loss adjustment expense (loss and LAE)	(172,273)	(132,396)	(487,759)	(395,718)
Commission and other acquisition expenses	(54,810)	(39,868)	(159,744)	(139,895)
General and administrative expenses	(8,595)	(9,038)	(25,819)	(26,717)
Underwriting (loss) income	$ (15,677)	$ (3,816)	$ (41,932)	$ (15,482)
Reconciliation to net income
Net loss and LAE ratio	78.30%	74.60%	77.20%	72.40%
Commission and other acquisition expense ratio	24.90%	22.50%	25.30%	25.60%
General and administrative expense ratio	3.90%	5.10%	4.10%	4.80%
Expense ratio	28.80%	27.60%	29.40%	30.40%
Combined ratio	107.10%	102.20%	106.60%	102.80%
Assets [Abstract]
Total assets - reportable segments	$ 1,898,036		$ 1,898,036		1,787,320
Corporate assets	0		0		0
Total assets	1,898,036		1,898,036		1,787,320
Operating segments [Member] | AmTrust Reinsurance segment [Member]
Net Income (Loss) [Abstract]
Gross premiums written	420,019	$ 520,104	1,575,677	$ 1,591,902
Net premiums written	410,193	511,561	1,529,980	1,507,389
Net premiums earned	436,353	523,137	1,450,811	1,413,699
Other insurance revenue	0	0	0	0
Net loss and loss adjustment expense (loss and LAE)	(355,030)	(334,310)	(1,047,222)	(898,703)
Commission and other acquisition expenses	(138,650)	(166,836)	(465,789)	(447,604)
General and administrative expenses	(771)	(759)	(2,240)	(2,308)
Underwriting (loss) income	$ (58,098)	$ 21,232	$ (64,440)	$ 65,084
Reconciliation to net income
Net loss and LAE ratio	81.40%	63.90%	72.20%	63.50%
Commission and other acquisition expense ratio	31.70%	31.90%	32.10%	31.70%
General and administrative expense ratio	0.20%	0.10%	0.10%	0.20%
Expense ratio	31.90%	32.00%	32.20%	31.90%
Combined ratio	113.30%	95.90%	104.40%	95.40%
Assets [Abstract]
Total assets - reportable segments	$ 4,362,008		$ 4,362,008		3,900,067
Corporate assets	0		0		0
Total assets	4,362,008		4,362,008		$ 3,900,067
Other [Member]
Net Income (Loss) [Abstract]
Gross premiums written	0	$ 0	81	$ 0
Net premiums written	0	0	90	0
Net premiums earned	0	0	90	0
Other insurance revenue	0	0	0	0
Net loss and loss adjustment expense (loss and LAE)	(8,665)	(45)	(10,176)	(2,940)
Commission and other acquisition expenses	(2)	(2)	3	(2)
General and administrative expenses	0	0	0	0
Underwriting (loss) income	$ (8,667)	$ (47)	$ (10,083)	$ (2,942)