Reserve for Losses and Loss Adjustment Expenses - Reconciliation of beginning and ending balances of losses and loss adjustment reserves (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Reserve for losses and loss adjustment expenses at beginning of year		$ 17,757,156	$ 16,513,929	$ 13,891,842
Unpaid losses and loss adjustment expenses recoverable		5,599,231	4,314,855	4,082,650
Net reserve for losses and loss adjustment expenses at beginning of year		12,157,925	12,199,074	9,809,192
Net incurred losses and loss adjustment expenses relating to losses occurring in:
Current year		5,798,528	4,940,987	4,851,051
Prior years		(771,011)	(356,184)	(161,452)
Total net incurred losses and loss adjustment expenses		5,027,517	4,584,803	4,689,599
Net losses and loss adjustment expense reserves of acquired business	[1]	0	104,176	0
Retroactive reinsurance transaction	[2]	0	(444,147)	182,210
Impact of deconsolidation of Somers		0	(1,460,611)	0
Foreign exchange (gains) losses and other		(290,376)	1,181	179,190
Net paid losses and loss adjustment expenses relating to losses occurring in:
Current year		(887,464)	(734,846)	(661,529)
Prior years		(2,254,484)	(2,091,705)	(1,999,588)
Total net paid losses and loss adjustment expenses		(3,141,948)	(2,826,551)	(2,661,117)
Net reserve for losses and loss adjustment expenses at end of year		13,753,118	12,157,925	12,199,074
Unpaid losses and loss adjustment expenses recoverable		6,278,825	5,599,231	4,314,855
Reserve for losses and loss adjustment expenses at end of year		$ 20,031,943	$ 17,757,156	$ 16,513,929

[1] Represents activity related to the Company’s acquisitions in the 2021 period.

[2] See ‘Retroactive Reinsurance Transactions’ section.