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		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Reserve for losses and loss adjustment expenses at beginning of period		$ 16,443,952	$ 14,309,580	$ 16,513,929	$ 13,891,842
Unpaid losses and loss adjustment expenses recoverable		3,916,650	4,070,114	4,314,855	4,082,650
Net reserve for losses and loss adjustment expenses at beginning of period		12,527,302	10,239,466	12,199,074	9,809,192
Net incurred losses and loss adjustment expenses relating to losses occurring in:
Current year		1,219,081	1,274,589	2,463,853	2,409,031
Prior years		(59,250)	(44,067)	(100,922)	(63,090)
Total net incurred losses and loss adjustment expenses		1,159,831	1,230,522	2,362,931	2,345,941
Retroactive reinsurance transaction	[1]	0	0	(183,893)	60,635
Net foreign exchange (gains) losses		135,847	51,157	88,970	(91,416)
Net paid losses and loss adjustment expenses relating to losses occurring in:
Current year		(164,441)	(128,174)	(223,425)	(169,434)
Prior years		(607,911)	(504,254)	(1,193,029)	(1,066,201)
Total net paid losses and loss adjustment expenses		(772,352)	(632,428)	(1,416,454)	(1,235,635)
Net reserve for losses and loss adjustment expenses at end of period		13,050,628	10,888,717	13,050,628	10,888,717
Unpaid losses and loss adjustment expenses recoverable		4,146,020	4,156,157	4,146,020	4,156,157
Reserve for losses and loss adjustment expenses at end of period		$ 17,196,648	$ 15,044,874	$ 17,196,648	$ 15,044,874

[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”). During the 2020 first quarter, the Company entered into a reinsurance to close agreement of the 2017 and prior years of account previously covered by a third party arrangement.