Losses and Loss Expenses - Reconciliation of the beginning and ending balances of unpaid losses and losee expenses (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2017	Sep. 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		6,272,852	4,702,321
Prior year		(93,590)	(195,647)
Total net incurred losses and loss expenses		6,179,262	4,506,674
Foreign exchange and other		555,350	(263,342)
Less net losses and loss expenses paid in respect of losses occurring in:
Current year		654,494	695,256
Prior year		3,505,086	3,139,995
Total net paid losses		4,159,580	3,835,251
Net unpaid losses and loss expenses, end of period		23,034,303	20,700,235
Unpaid losses and loss expenses recoverable, end of period		6,962,787
Unpaid losses and loss expenses, end of period		29,988,337	26,170,383
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]	$ 6,954,034	$ 5,470,148

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