Related Party Transactions - AmTrust (Details) $ in Thousands														1 Months Ended		3 Months Ended		9 Months Ended
	Aug. 31, 2019 USD ($)	Jul. 31, 2019 USD ($)	Jan. 24, 2019	Jan. 11, 2019 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jul. 01, 2018	Jan. 01, 2018	Jul. 01, 2016	Jan. 01, 2012 EUR (€)	Apr. 01, 2011 EUR (€)	Jun. 11, 2008	Jul. 01, 2007	Jan. 31, 2019 USD ($)	Jan. 31, 2019 EUR (€)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 18, 2017
Related Party Transaction [Line Items]
Loan to related party						$ 167,975										$ 167,975		$ 167,975
Interest income																$ 27,441	$ 35,477	$ 92,144	$ 104,793
Leah Karfunkel
Related Party Transaction [Line Items]
Noncontrolling interest, ownership percentage																8.10%		8.10%
Leah Karfunkel | National General Holdings Corporation
Related Party Transaction [Line Items]
Noncontrolling interest, ownership percentage																39.40%		39.40%
Barry Zyskind
Related Party Transaction [Line Items]
Noncontrolling interest, ownership percentage																7.60%		7.60%
George Karfunkel, less than 5% ownership
Related Party Transaction [Line Items]
Noncontrolling interest, ownership percentage																5.00%		5.00%
Founding shareholders | AmTrust Financial Services, Inc.
Related Party Transaction [Line Items]
Controlling interest, ownership percentage																53.60%		53.60%
AmTrust Financial Services, Inc. | London Interbank Offered Rate (LIBOR)
Related Party Transaction [Line Items]
Basis spread on variable rate																				2.00%
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Agreement
Related Party Transaction [Line Items]
Percent of premiums written, net of cost of unaffiliated inuring reinsurance from related party													40.00%
Percent of losses with respect to current lines of business, excluding those above covered business threshold to related party		40.00%					40.00%						40.00%
Commission rate, percent of ceded written premiums												34.375%	31.00%
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Agreement | Post-Termination Endorsement, Maximum Loss Corridor
Related Party Transaction [Line Items]
Related party transaction amount		$ 40,500
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Agreement | Returned Gross Unearned Premiums
Related Party Transaction [Line Items]
Related party transaction amount					$ (647,980)
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Agreement | Returned Gross Unearned Premiums | AmTrust International Insurance, Ltd. (AII)
Related Party Transaction [Line Items]
Related party transaction amount																$ 43,240
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Agreement | Returned Unearned Premiums, Net
Related Party Transaction [Line Items]
Related party transaction amount														$ (436,760)
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Agreement | Cash And Investments Transferred Due To Unearned Premiums
Related Party Transaction [Line Items]
Related party transaction amount														(480,000)
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Agreement | Funds Withheld Arrangement
Related Party Transaction [Line Items]
Related party transaction amount				$ (575,000)														$ 575,000
Accrued interest on collateral held by related party																5,073		5,073
Calculated interest rate with related party				3.50%
Interest income from related party																5,073		14,500
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Agreement | AmTrust Quota Shares Reinsurance Agreement, Partial Termination Amendment
Related Party Transaction [Line Items]
Percent of ceding commission payable, period increase						5.00%
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Agreement | Post-Termination Endorsement
Related Party Transaction [Line Items]
Related party transaction, percentage of required funding on obligations		105.00%
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Agreement | Post-Termination Endorsement, Minimum Excess Funding Requirement
Related Party Transaction [Line Items]
Related party transaction amount		$ 54,000
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Agreement | Minimum
Related Party Transaction [Line Items]
Commission rate, adjustment criteria, loss ratio floor							81.50%
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Agreement | Maximum
Related Party Transaction [Line Items]
Commission rate, adjustment criteria, loss ratio ceiling							95.00%
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Agreement | Maximum | Post-Termination Endorsement
Related Party Transaction [Line Items]
Related party transaction, percentage of required funding on obligations		110.00%
AmTrust Financial Services, Inc. | European Hospital Liability Quota Share Agreement
Related Party Transaction [Line Items]
Commission rate, percent of ceded written premiums											5.00%
Premiums and losses related to policies, percentage									32.50%		40.00%
Maximum limit of liability attaching (in EUR) | €										€ 10,000,000	€ 5,000,000
Percent basis currency equivalent of maximum limit of liability attaching											100.00%
Future premiums and losses related to policies, percentage									20.00%
AmTrust Financial Services, Inc. | Reinsurer Trust Assets Collateral Agreement
Related Party Transaction [Line Items]
Collateral held by related party						$ 3,650,418										1,354,283		1,354,283
Accrued interest on collateral held by related party						23,283										7,645		7,645
AmTrust Financial Services, Inc. | Reinsurance Brokerage Agreements
Related Party Transaction [Line Items]
Brokerage fee, percent of premium assumed													1.25%
AmTrust Financial Services, Inc. | Reinsurance Brokerage Agreements | Brokerage expense
Related Party Transaction [Line Items]
Expenses from related party transactions																930	6,145	4,305	18,408
Balance due to related party						14,199										2,835		2,835
AmTrust Financial Services, Inc. | Asset Management Agreement
Related Party Transaction [Line Items]
Cancellation notice period													30 days
Quarterly brokerage fee, above portfolio threshold, percent of average holdings								0.02125%					0.0375%
AmTrust Financial Services, Inc. | Asset Management Agreement | Investment Management Fee
Related Party Transaction [Line Items]
Expenses from related party transactions																$ 618	1,055	$ 2,071	3,137
AmTrust Financial Services, Inc. | National General Holdings Corporation
Related Party Transaction [Line Items]
Noncontrolling interest, ownership percentage																1.50%		1.50%
AEL | AmTrust Quota Share Reinsurance Agreement
Related Party Transaction [Line Items]
Percentage ceded							20.00%
AEL | AmTrust European Hospital Liability Quota Share Agreement
Related Party Transaction [Line Items]
Collateral held by related party						249,948										$ 240,386		$ 240,386
Accrued interest on collateral held by related party						$ 1,976										1,305		1,305
AmTrust International Underwriters DAC (AIU) | AmTrust European Hospital Liability Quota Share Agreement | Collateral Provided For Share Under Quota Share Agreement
Related Party Transaction [Line Items]
Related party transaction amount														$ (51,244)	€ (45,113,000)			55,701
Calculated interest rate with related party			0.50%
Interest income from related party																71		196
AmTrust International Underwriters DAC (AIU) | AmTrust European Hospital Liability Quota Share Agreement | Accrued Interest On Collateral Provided For Share Under Quota Share Agreement
Related Party Transaction [Line Items]
Related party transaction amount																		192
Risk Services - Vermont, Inc. | Insurance Management Services Agreement
Related Party Transaction [Line Items]
Cancellation notice period	3 months
Agreement initial term	3 years
Renewal term	3 years
Risk Services - Vermont, Inc. | Insurance Management Services Agreement, Initial Retainer
Related Party Transaction [Line Items]
Expenses from related party transactions	$ 100
Risk Services - Vermont, Inc. | Insurance Management Services Agreement, Fees
Related Party Transaction [Line Items]
Expenses from related party transactions	$ 100															100		100
Notes Receivable, Related Party
Related Party Transaction [Line Items]
Interest income																$ 1,777	$ 1,658	$ 5,441	$ 4,651
Effective yield																4.20%	3.90%	4.30%	3.70%