Related Party Transactions and Major Customers (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Related Party Transaction [Line Items]
Gross premiums written (Note 6)	$ 9,213,540	$ 7,833,798	$ 5,806,165
Gross premiums earned	7,079,632	6,257,814	5,078,682
Acquisition expenses	1,568,606	1,214,858	897,677
Net reinsurance costs and recoveries	(1,512,300)	(2,029,529)	(968,595)
Total reserve for claims and claim expenses	15,892,573	13,294,630	10,381,138
Equity method investment, dividends and distributions	$ 24,200	$ 33,900	$ 30,000
Gross premiums written | Three Brokers | Customer Concentration Risk
Related Party Transaction [Line Items]
Percent of gross premiums received	82.20%	78.00%	79.60%
Gross premiums written | Aon plc | Customer Concentration Risk
Related Party Transaction [Line Items]
Percent of gross premiums received	35.40%	35.80%	42.80%
Gross premiums written | Marsh & McLennan Companies, Inc. | Customer Concentration Risk
Related Party Transaction [Line Items]
Percent of gross premiums received	33.30%	30.00%	24.50%
Gross premiums written | Arthur J Gallagher | Customer Concentration Risk
Related Party Transaction [Line Items]
Percent of gross premiums received	13.50%	12.20%	12.30%
Tower Hill Companies
Related Party Transaction [Line Items]
Gross premiums written (Note 6)	$ 82,000	$ 69,800	$ 55,500
Gross premiums earned	80,000	63,000	51,400
Acquisition expenses	15,800	11,300	7,900
Related party receivables	19,100	21,700
Net reinsurance costs and recoveries	68,600	28,500	13,200
Total reserve for claims and claim expenses	89,800	68,000
Equity method investment, dividends and distributions	10,500	15,000	9,500
Top Layer Reinsurance Ltd.
Related Party Transaction [Line Items]
Equity method investment, dividends and distributions	8,700	9,300	18,000
Top Layer Reinsurance Ltd. | Management Fee
Related Party Transaction [Line Items]
Revenue recorded	$ 2,400	$ 2,500	$ 2,400