Reinsurance - Narrative (Details) - USD ($)	Jul. 31, 2019	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable for unpaid claims and claims adjustments		$ 597,677,000	$ 623,422,000
Reinsurance recoverable for unpaid claims and claims adjustments, valuation allowance		0	0
Reinsurance retention policy, excess retention, amount reinsured	$ 2,178,535,000
Reinsurance retention policy, amount retained	600,000,000
Reinsurance recoverable for paid and unpaid claims and claims adjustments, adverse development cover	155,000,000
Reinsurance recoverable for paid and unpaid claims and claims adjustments, ceded cumulative losses	445,000,000
Reinsurance recoverables, including reinsurance premium paid		597,677,000	623,422,000	$ 615,481,000	$ 71,901,000
Deferred gain on retroactive reinsurance		79,995,000	112,950,000
Maximum
Reinsurance Retention Policy [Line Items]
Reinsurance recoverables on paid losses, gross	312,786,000
Diversified Reinsurance | Miscellaneous
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable for unpaid claims and claims adjustments		69,454,000	62,699,000
AmTrust Reinsurance
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable for unpaid claims and claims adjustments	$ 445,000,000
Reinsurance recoverables, including reinsurance premium paid		$ 524,995,000	$ 557,950,000