Reserve for Losses and Loss Adjustment Expenses - Reconciliation of beginning and ending balances of losses and loss adjustment reserves (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2022	Mar. 31, 2021
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Reserve for losses and loss adjustment expenses at beginning of period		$ 17,757,156	$ 16,513,929
Unpaid losses and loss adjustment expenses recoverable		5,599,231	4,314,855
Net reserve for losses and loss adjustment expenses at beginning of period		12,157,925	12,199,074
Net incurred losses and loss adjustment expenses relating to losses occurring in:
Current year		1,142,647	1,244,772
Prior years		(141,812)	(41,672)
Total net incurred losses and loss adjustment expenses		1,000,835	1,203,100
Retroactive reinsurance transaction	[1]	0	(183,893)
Net foreign exchange (gains) losses and other		(32,640)	(46,877)
Net paid losses and loss adjustment expenses relating to losses occurring in:
Current year		(70,806)	(58,984)
Prior years		(656,205)	(585,118)
Total net paid losses and loss adjustment expenses		(727,011)	(644,102)
Net reserve for losses and loss adjustment expenses at end of period		12,399,109	12,527,302
Unpaid losses and loss adjustment expenses recoverable		5,709,998	3,916,650
Reserve for losses and loss adjustment expenses at end of period		$ 18,109,107	$ 16,443,952

[1] During the 2021 first quarter, the Company entered into a reinsurance to close and other related agreements with Premia Managing Agency Limited (“Premia”), in connection with the 2018 and prior years of account related to the acquisition of Barbican Group Holdings Limited (“Barbican”).