Reserve for Loss and Loss Adjustment Expenses (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Dec. 31, 2019
Insurance [Abstract]
Reserve for reported loss and LAE			$ 1,194,204,000	$ 1,271,358,000
Reserve for losses incurred but not reported (IBNR)			1,054,841,000	1,168,549,000
Reserve for loss and LAE	$ 2,249,045,000	$ 3,050,271,000	$ 2,249,045,000	$ 2,439,907,000
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross loss and LAE reserves, January 1	2,439,907,000	3,126,134,000
Less: reinsurance recoverable on unpaid losses, January 1	623,422,000	71,901,000
Net loss and LAE reserves, January 1	1,816,485,000	3,054,233,000
Net incurred losses related to Current year	21,619,000	145,431,000
Net incurred losses related to Prior years	(533,000)	7,258,000
Net loss and loss adjustment expenses	21,086,000	152,689,000
Net paid losses related to Current year	(214,000)	(416,000)
Net paid losses related to Prior years	(193,430,000)	(219,950,000)
Net paid losses	(193,644,000)	(220,366,000)
Effect of foreign exchange rate movements	(15,764,000)	(8,260,000)
Net loss and LAE reserves, March 31	1,628,163,000	2,978,296,000
Reinsurance recoverable on unpaid losses, March 31	620,882,000	71,975,000
Gross loss and LAE reserves, March 31	2,249,045,000	3,050,271,000
Segment Reporting Information [Line Items]
Net adverse prior year development (net favorable prior year reserve development)	533,000	(7,258,000)
Other
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net loss and loss adjustment expenses		228,000
Segment Reporting Information [Line Items]
Net adverse prior year development (net favorable prior year reserve development)		(228,000)
Diversified Reinsurance
Segment Reporting Information [Line Items]
Net adverse prior year development (net favorable prior year reserve development)	533,000	1,096,000
AmTrust Reinsurance
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Less: reinsurance recoverable on unpaid losses, January 1	557,950,000
Reinsurance recoverable on unpaid losses, March 31	557,950,000
Segment Reporting Information [Line Items]
Net adverse prior year development (net favorable prior year reserve development)	$ 0	$ (8,126,000)