Losses and Loss Expenses - Reconciliation of the beginning and ending balances of unpaid losses and losee expenses (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss expenses, beginning of period		$ 25,939,571	$ 25,439,744
Unpaid losses and loss expenses recoverable, beginning of period		5,491,297
Net unpaid losses and loss expenses, beginning of period		20,459,271	20,190,839
Acquired reserves		0	101,315
Increase (decrease) in net losses and loss expenses incurred in respect of losses occurring in:
Current year		3,175,267	3,156,913
Prior year		(62,728)	(142,042)
Total net incurred losses and loss expenses		3,112,539	3,014,871
Foreign exchange and other		292,040	(286,488)
Less net losses and loss expenses paid in respect of losses occurring in:
Current year		343,934	336,223
Prior year		2,547,073	2,078,836
Total net paid losses		2,891,007	2,415,059
Net unpaid losses and loss expenses, end of period		20,972,843	20,605,478
Unpaid losses and loss expenses recoverable, end of period		5,858,369
Unpaid losses and loss expenses, end of period		26,822,346	26,020,564
Total P&C
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss expenses recoverable, beginning of period	[1]	5,480,300	5,248,905
Less net losses and loss expenses paid in respect of losses occurring in:
Unpaid losses and loss expenses recoverable, end of period	[1]	$ 5,849,503	$ 5,415,086

[1] Property and Casualty business only, net of provision for uncollectible reinsurance.