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Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Concentration Risk [Line Items]
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses (net of allowance for credit losses: $11,636 and $1,364)		$ 4,500,802	$ 4,346,816
Concentration risk percentage		10.00%
Stockholders' equity | Reinsurer concentration risk
Concentration Risk [Line Items]
Concentration risk percentage		1.80%	1.70%
AM Best A minus Or Better Rating | Reinsurance recoverable | Credit concentration risk
Concentration Risk [Line Items]
Concentration risk percentage		63.90%	61.20%
No AM Best rating | Reinsurance recoverable | Credit concentration risk
Concentration Risk [Line Items]
Concentration risk percentage	[1]	36.10%	38.80%
No AM Best rating | Reinsurance recoverable | Credit concentration risk | Reinsurance trusts or letters of credit
Concentration Risk [Line Items]
Concentration risk percentage		94.00%
[1] Over 94% of such amount is collateralized through reinsurance trusts, funds withheld arrangements, letters of credit or other

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Amount, after valuation allowance, recoverable under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, policy benefits and policy reserves. Excludes premiums paid under reinsurance contracts.

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