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Commitments and Contingencies - Concentrations of credit risk (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Concentration risk percentage	10.00%
Customer Concentration Risk | Gross Written Premiums | Marsh And McLennan Companies
Concentration Risk [Line Items]
Concentration risk percentage	13.30%	9.60%	9.30%
Customer Concentration Risk | Gross Written Premiums | AON Corporation and Subsidiaries
Concentration Risk [Line Items]
Concentration risk percentage	12.00%	12.20%	11.40%

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

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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