Reserve for Loss and Loss Adjustment Expenses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Dec. 31, 2018
Insurance [Abstract]
Reserve for reported loss and LAE			$ 1,548,160	$ 1,571,217
Reserve for losses incurred but not reported (IBNR)			1,431,953	1,484,759
Reserve for loss and LAE	$ 2,980,113	$ 2,386,722	$ 2,980,113	$ 3,055,976
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross loss and LAE reserves, January 1	3,055,976	2,386,722
Less: reinsurance recoverable on unpaid losses, January 1	1,743	24,883
Net loss and LAE reserves, January 1	3,054,233	2,361,839
Net incurred losses related to Current year	145,431	343,421
Net incurred losses related to Prior years	7,258	9,785
Net loss and loss adjustment expenses	152,689	353,206
Net paid losses related to Current year	(416)	(44,317)
Net paid losses related to Prior years	(219,950)	(218,176)
Net paid losses	(220,366)	(262,493)
Effect of foreign exchange rate movements	(8,260)	13,960
Net loss and LAE reserves, March 31	2,978,296	2,466,512
Reinsurance recoverable on unpaid losses, March 31	1,817	25,490
Gross loss and LAE reserves, March 31	2,980,113	2,492,002
Segment Reporting Information [Line Items]
Adverse development, net	7,258	9,785
Other
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net loss and loss adjustment expenses	228	0
Segment Reporting Information [Line Items]
Adverse development, net	228	0
Diversified Reinsurance
Segment Reporting Information [Line Items]
Adverse development, net	(1,096)	1,266
AmTrust Reinsurance
Segment Reporting Information [Line Items]
Adverse development, net	$ 8,126	$ 8,519