Related Party Transactions - AmTrust Narrative (Details) $ in Thousands														3 Months Ended			9 Months Ended		12 Months Ended
	Mar. 16, 2020 USD ($)	Aug. 31, 2019 USD ($)	Jul. 31, 2019 USD ($)	Jan. 24, 2019	Jan. 11, 2019 USD ($)	Dec. 31, 2018	Jan. 01, 2018	Jul. 01, 2016	Jul. 01, 2013	Jan. 01, 2012 EUR (€)	Apr. 01, 2011 EUR (€)	Jun. 11, 2008	Jul. 01, 2007	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Jul. 01, 2018	Dec. 18, 2017
Related Party Transaction [Line Items]
Loan to related party														$ 167,975			$ 167,975		$ 167,975
Interest income														$ 13,062	$ 27,441		$ 46,090	$ 92,144
Founding shareholders | AmTrust Financial Services, Inc.
Related Party Transaction [Line Items]
Controlling interest, ownership percentage														53.30%			53.30%
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 Segment
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 Segment | 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 Segment | Returned Gross Unearned Premiums
Related Party Transaction [Line Items]
Related party transaction amount																$ (647,980)
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Segment | Returned Unearned Premiums, Net
Related Party Transaction [Line Items]
Related party transaction amount																$ (436,760)
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Segment | Investments transferred out for transactions under remaining AmTrust Quota Share business
Related Party Transaction [Line Items]
Related party transaction amount					$ (575,000)												$ 575,000		575,000
Accrued interest on collateral held by related party														$ 3,845			$ 3,845		5,073
Calculated interest rate with related party					3.50%									2.65%			2.65%
Interest income from related party														$ 3,845	5,073		$ 11,451	14,500
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Segment | Investments transferred out related to Partial Termination Amendment and Commutation
Related Party Transaction [Line Items]
Percent of ceding commission payable, period increase						5.00%
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Segment | 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 Segment | 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 Segment | Post-Termination Endorsement No. 2
Related Party Transaction [Line Items]
Related party transaction, percentage of required funding on obligations	110.00%
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Segment | Post-Termination Endorsement No. 2, Minimum Excess Funding Requirement
Related Party Transaction [Line Items]
Related party transaction amount	$ 54,000
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Segment | Minimum
Related Party Transaction [Line Items]
Commission rate, adjustment criteria, loss ratio floor									81.50%
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Segment | Maximum
Related Party Transaction [Line Items]
Commission rate, adjustment criteria, loss ratio ceiling									95.00%
AmTrust Financial Services, Inc. | AmTrust Quota Share Reinsurance Segment | 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. | European Hospital Liability Quota Share Agreement | Maximum | Post-Termination Endorsement No. 1
Related Party Transaction [Line Items]
Related party transaction, percentage of required funding on obligations	120.00%
Solvency ratio	100.00%
Collateral on exposure	100.00%
AmTrust Financial Services, Inc. | Reinsurer Trust Assets Collateral Agreement
Related Party Transaction [Line Items]
Collateral held by related party														759,348			759,348		1,155,955
Accrued interest on collateral held by related party														3,184			3,184		7,366
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														162	930		515	4,305
Balance due to related party														1,728			1,728		2,372
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%
AmTrust Financial Services, Inc. | Asset Management Agreement | Investment Management Fee
Related Party Transaction [Line Items]
Expenses from related party transactions														321	618		1,071	2,071
AEL | AmTrust Quota Share Reinsurance Segment
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														302,733			302,733		253,631
Accrued interest on collateral held by related party														1,883			1,883		1,821
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																	26,902		57,305
Calculated interest rate with related party				0.50%
Interest income from related party														64	71		262	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																	271		$ 269
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													100			100
Risk Services - Vermont, Inc. | Insurance Management Services Agreement, Fees
Related Party Transaction [Line Items]
Expenses from related party transactions		$ 100												25			75
Notes Receivable, Related Party
Related Party Transaction [Line Items]
Interest income														$ 886	$ 1,777		$ 3,111	$ 5,441
Effective yield														2.10%	4.20%		2.50%	4.30%
Maiden Holdings, Ltd. | Leah Karfunkel
Related Party Transaction [Line Items]
Noncontrolling interest, ownership percentage														7.90%			7.90%
Maiden Holdings, Ltd. | Barry Zyskind
Related Party Transaction [Line Items]
Noncontrolling interest, ownership percentage														7.40%			7.40%
Maiden Holdings, Ltd. | George Karfunkel, less than 5% ownership
Related Party Transaction [Line Items]
Noncontrolling interest, ownership percentage														5.00%			5.00%