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		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Liability for Future Policy Benefits and Unpaid Claims and Claims Adjustment Expense [Abstract]
Reserve for losses and loss adjustment expenses at beginning of period		$ 15,044,874	$ 12,230,316	$ 13,891,842	$ 11,853,297
Unpaid losses and loss adjustment expenses recoverable		4,156,157	3,024,797	4,082,650	2,814,291
Net reserve for losses and loss adjustment expenses at beginning of period		10,888,717	9,205,519	9,809,192	9,039,006
Net incurred losses and loss adjustment expenses relating to losses occurring in:
Current year		1,264,315	855,352	3,673,346	2,419,044
Prior years		(48,042)	(52,897)	(111,132)	(130,514)
Total net incurred losses and loss adjustment expenses		1,216,273	802,455	3,562,214	2,288,530
Retroactive reinsurance transaction	[1]	0	0	60,635	(225,500)
Net foreign exchange (gains) losses		114,122	(73,200)	22,706	(74,981)
Net paid losses and loss adjustment expenses relating to losses occurring in:
Current year		(189,961)	(175,611)	(359,395)	(301,099)
Prior years		(512,263)	(399,948)	(1,578,464)	(1,366,741)
Total net paid losses and loss adjustment expenses		(702,224)	(575,559)	(1,937,859)	(1,667,840)
Net reserve for losses and loss adjustment expenses at end of period		11,516,888	9,359,215	11,516,888	9,359,215
Unpaid losses and loss adjustment expenses recoverable		4,383,638	3,030,169	4,383,638	3,030,169
Reserve for losses and loss adjustment expenses at end of period		$ 15,900,526	$ 12,389,384	$ 15,900,526	$ 12,389,384

[1] During 2020 first quarter, a subsidiary of the Company entered into a reinsurance to close agreement of the 2017 and prior years of account previously covered by a third party arrangement, while in the 2019 first quarter, a subsidiary of the Company entered into a retroactive reinsurance transaction with third party reinsurer to reinsure run-off liabilities associated with certain U.S. insurance exposures.