Segment Information - Net Income (Loss) and Total Assets (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | Segment									2
Income Statement [Abstract]
Gross premiums written									$ 2,816,051	$ 2,831,348	$ 2,662,825
Net premiums written									2,761,988	2,654,952	2,514,116
Net premiums earned									2,732,779	2,568,150	2,429,069
Other insurance revenue									9,802	10,817	11,512
Net loss and LAE									(2,160,011)	(1,819,906)	(1,633,570)
Commission and other acquisition expenses									(820,758)	(773,664)	(724,197)
General and administrative expenses									(38,869)	(38,577)	(38,328)
Underwriting loss									(277,057)	(53,180)	44,486
Reconciliation to net loss
Net investment income and realized gains on investment									178,567	152,666	133,590
Net impairment losses recognized in earnings									0	0	(1,060)
Interest and amortization expenses									(23,260)	(28,173)	(29,063)
Accelerated amortization of senior note issuance cost									(2,809)	(2,345)	0
Amortization of intangible assets									(2,132)	(2,461)	(2,840)
Foreign exchange gains (losses)									(14,921)	11,612	7,753
Other general and administrative expenses									(31,691)	(28,407)	(26,544)
Income tax benefit (expense)									3,558	(1,574)	(2,038)
Net (loss) income	$ (124,857)	$ (55,054)	$ (16,335)	$ 26,501	$ (69,374)	$ 40,796	$ 39,887	$ 36,829	$ (169,745)	$ 48,138	$ 124,284
Net loss and LAE ratio									78.80%	70.60%	66.90%
Commission and other acquisition expense ratio									29.90%	30.00%	29.70%
General and administrative expense ratio									2.60%	2.60%	2.70%
Expense ratio									32.50%	32.60%	32.40%
Combined ratio									111.30%	103.20%	99.30%
Assets [Abstract]
Reinsurance balances receivable, net	345,043				410,166				$ 345,043	$ 410,166
Reinsurance recoverable on unpaid losses	117,611				99,936				117,611	99,936	$ 71,248	$ 75,873
Deferred commission and other acquisition expenses	439,597				424,605				439,597	424,605
Loan to related party	167,975				167,975				167,975	167,975
Goodwill and intangible assets, net	75,583				77,715				75,583	77,715	81,920
Restricted cash and cash equivalents and investments	4,868,524				4,398,796				4,868,524	4,398,796
Other assets	123,113				148,912				123,113	148,912
Total assets	6,644,189				6,252,299				6,644,189	6,252,299
AmTrust Reinsurance segment
Income Statement [Abstract]
Gross premiums written									1,993,478	2,006,646	1,885,974
Operating Segments
Assets [Abstract]
Reinsurance balances receivable, net	345,043				400,224				345,043	400,224
Reinsurance recoverable on unpaid losses	67,459				87,232				67,459	87,232
Deferred commission and other acquisition expenses	439,597				424,605				439,597	424,605
Loan to related party	167,975				167,975				167,975	167,975
Goodwill and intangible assets, net	75,583				77,715				75,583	77,715
Restricted cash and cash equivalents and investments	4,826,920				4,385,603				4,826,920	4,385,603
Other assets	108,939				144,033				108,939	144,033
Total assets - reportable segments	6,031,516				5,687,387				6,031,516	5,687,387
Corporate assets	612,673				564,912				612,673	564,912
Total assets	6,644,189				6,252,299				6,644,189	6,252,299
Operating Segments | Diversified Reinsurance segment
Income Statement [Abstract]
Gross premiums written									822,777	824,341	776,852
Net premiums written									807,362	766,119	734,781
Net premiums earned									823,365	724,124	744,875
Other insurance revenue									9,802	10,817	11,512
Net loss and LAE									(650,916)	(579,520)	(547,296)
Commission and other acquisition expenses									(205,982)	(188,506)	(196,292)
General and administrative expenses									(35,817)	(35,681)	(35,312)
Underwriting loss									$ (59,548)	$ (68,766)	$ (22,513)
Reconciliation to net loss
Net loss and LAE ratio									78.10%	78.90%	72.30%
Commission and other acquisition expense ratio									24.70%	25.60%	26.00%
General and administrative expense ratio									4.30%	4.90%	4.70%
Expense ratio									29.00%	30.50%	30.70%
Combined ratio									107.10%	109.40%	103.00%
Assets [Abstract]
Reinsurance balances receivable, net	294,775				281,073				$ 294,775	$ 281,073
Reinsurance recoverable on unpaid losses	44,628				54,299				44,628	54,299
Deferred commission and other acquisition expenses	79,633				85,432				79,633	85,432
Loan to related party	0				0				0	0
Goodwill and intangible assets, net	75,583				77,715				75,583	77,715
Restricted cash and cash equivalents and investments	1,250,736				1,247,793				1,250,736	1,247,793
Other assets	27,554				41,008				27,554	41,008
Total assets - reportable segments	1,772,909				1,787,320				1,772,909	1,787,320
Corporate assets	0				0				0	0
Total assets	1,772,909				1,787,320				1,772,909	1,787,320
Operating Segments | AmTrust Reinsurance segment
Income Statement [Abstract]
Gross premiums written									1,993,478	2,006,646	$ 1,885,974
Net premiums written									1,954,856	1,888,428	1,779,334
Net premiums earned									1,909,644	1,843,621	1,684,191
Other insurance revenue									0	0	0
Net loss and LAE									(1,498,881)	(1,225,830)	(1,074,072)
Commission and other acquisition expenses									(614,777)	(584,820)	(527,863)
General and administrative expenses									(3,052)	(2,896)	(3,016)
Underwriting loss									$ (207,066)	$ 30,075	$ 79,240
Reconciliation to net loss
Net loss and LAE ratio									78.40%	66.50%	63.80%
Commission and other acquisition expense ratio									32.20%	31.70%	31.30%
General and administrative expense ratio									0.20%	0.20%	0.20%
Expense ratio									32.40%	31.90%	31.50%
Combined ratio									110.80%	98.40%	95.30%
Assets [Abstract]
Reinsurance balances receivable, net	50,268				119,151				$ 50,268	$ 119,151
Reinsurance recoverable on unpaid losses	22,831				32,933				22,831	32,933
Deferred commission and other acquisition expenses	359,964				339,173				359,964	339,173
Loan to related party	167,975				167,975				167,975	167,975
Goodwill and intangible assets, net	0				0				0	0
Restricted cash and cash equivalents and investments	3,576,184				3,137,810				3,576,184	3,137,810
Other assets	81,385				103,025				81,385	103,025
Total assets - reportable segments	4,258,607				3,900,067				4,258,607	3,900,067
Corporate assets	0				0				0	0
Total assets	$ 4,258,607				$ 3,900,067				4,258,607	3,900,067
Other
Income Statement [Abstract]
Gross premiums written									(204)	361	$ (1)
Net premiums written									(230)	405	1
Net premiums earned									(230)	405	3
Other insurance revenue									0	0	0
Net loss and LAE									(10,214)	(14,556)	(12,202)
Commission and other acquisition expenses									1	(338)	(42)
General and administrative expenses									0	0	0
Underwriting loss									$ (10,443)	$ (14,489)	$ (12,241)