Segment Information - Net Income (Loss) and Total Assets (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) Segment	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of reporting segments | Segment	2
Net Income (Loss) [Abstract]
Gross premiums written	$ 11,734	$ (561,139)
Net premiums written	10,372	(561,530)
Net premiums earned	31,215	183,102
Other insurance revenue	408	812
Net loss and loss adjustment expenses (loss and LAE)	(21,086)	(152,689)
Commission and other acquisition expenses	(11,973)	(69,617)
General and administrative expenses	(2,257)	(4,297)
Underwriting loss	(3,693)	(42,689)
Reconciliation to net income from continuing operations
Net investment income and realized gains on investment	29,002	20,921
Total other-than-temporary impairment losses	(1,506)	0
Interest and amortization expenses	(4,831)	(4,829)
Foreign exchange and other gains, net	8,197	4,979
Other general and administrative expenses	(6,293)	(12,322)
Income tax (expense) benefit	(15)	38
Income (loss) from continuing operations	$ 20,861	$ (33,902)
Net loss and LAE ratio	66.70%	83.00%
Commission and other acquisition expense ratio	37.90%	37.90%
General and administrative expense ratio	27.00%	9.00%
Expense ratio	64.90%	46.90%
Combined ratio	131.60%	129.90%
Assets [Abstract]
Total assets	$ 3,321,757		$ 3,568,196
Operating segments
Assets [Abstract]
Total assets - reportable segments	2,806,757		3,011,647
Corporate assets	515,000		556,549
Total assets	3,321,757		3,568,196
Operating segments | Diversified Reinsurance
Net Income (Loss) [Abstract]
Gross premiums written	11,734	$ 15,338
Net premiums written	10,372	14,947
Net premiums earned	12,531	25,292
Other insurance revenue	408	812
Net loss and loss adjustment expenses (loss and LAE)	(7,041)	(14,391)
Commission and other acquisition expenses	(4,979)	(9,261)
General and administrative expenses	(1,613)	(3,031)
Underwriting loss	$ (694)	$ (579)
Reconciliation to net income from continuing operations
Net loss and LAE ratio	54.40%	55.10%
Commission and other acquisition expense ratio	38.50%	35.50%
General and administrative expense ratio	12.50%	11.60%
Expense ratio	51.00%	47.10%
Combined ratio	105.40%	102.20%
Assets [Abstract]
Total assets - reportable segments	$ 163,684		167,845
Corporate assets	0		0
Total assets	163,684		167,845
Operating segments | AmTrust Reinsurance
Net Income (Loss) [Abstract]
Gross premiums written	0	$ (576,477)
Net premiums written	0	(576,477)
Net premiums earned	18,684	157,810
Other insurance revenue	0	0
Net loss and loss adjustment expenses (loss and LAE)	(14,045)	(138,070)
Commission and other acquisition expenses	(6,994)	(60,356)
General and administrative expenses	(644)	(1,266)
Underwriting loss	$ (2,999)	$ (41,882)
Reconciliation to net income from continuing operations
Net loss and LAE ratio	75.20%	87.50%
Commission and other acquisition expense ratio	37.40%	38.20%
General and administrative expense ratio	3.50%	0.80%
Expense ratio	40.90%	39.00%
Combined ratio	116.10%	126.50%
Assets [Abstract]
Total assets - reportable segments	$ 2,643,073		2,843,802
Corporate assets	0		0
Total assets	$ 2,643,073		$ 2,843,802
Other
Net Income (Loss) [Abstract]
Gross premiums written		$ 0
Net premiums written		0
Net premiums earned		0
Other insurance revenue		0
Net loss and loss adjustment expenses (loss and LAE)		(228)
Commission and other acquisition expenses		0
General and administrative expenses		0
Underwriting loss		$ (228)