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Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($)
$ in Thousands

12 Months Ended

Dec. 31, 2021

Dec. 31, 2020

Dec. 31, 2019

Concentration Risk [Line Items]

Reinsurance recoverable for paid and unpaid claims and claims adjustments

$ 5,880,735

$ 4,500,802

$ 4,346,816

Shareholders' equity | Reinsurer concentration risk | Largest balance due from any one carrier

Concentration Risk [Line Items]

Concentration risk percentage

^[1]

6.70%

1.80%

AM Best "A-" Or Better Rating | Reinsurance recoverable | Reinsurer concentration risk

Concentration Risk [Line Items]

Concentration risk percentage

69.70%

63.90%

Rating Below A- | Reinsurance recoverable | Reinsurer concentration risk | All Other Carriers

Concentration Risk [Line Items]

Concentration risk percentage

0.10%

0.10%

No AM Best rating | Reinsurance recoverable | Reinsurer concentration risk | All Other Carriers

Concentration Risk [Line Items]

Concentration risk percentage

^[2]

30.20%

36.00%

No AM Best rating | Reinsurance recoverable | Reinsurer concentration risk | Reinsurance trusts or letters of credit

Concentration Risk [Line Items]

Concentration risk percentage

91.00%

94.00%

[1]	The 2021 period included impact of deconsolidation of Somers.
[2]	At December 31, 2021 and 2020 period, over 91% and 94% of such amount is collateralized through reinsurance trusts, funds withheld arrangements, letters of credit or other, respectively

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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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