Losses and Loss Expenses - Reconciliation of the beginning and ending balances of unpaid losses and losee expenses (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 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		1,559,452	1,425,897
Prior year		24,004	(43,412)
Total net incurred losses and loss expenses		1,583,456	1,382,485
Foreign exchange and other		56,244	32,268
Less net losses and loss expenses paid in respect of losses occurring in:
Current year		106,472	104,681
Prior year		1,222,119	1,067,056
Total net paid losses		1,328,591	1,171,737
Net unpaid losses and loss expenses, end of period		20,770,380	20,535,170
Unpaid losses and loss expenses recoverable, end of period		5,685,623
Unpaid losses and loss expenses, end of period		26,445,124	25,913,484
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]	$ 5,674,744	$ 5,378,314

[1] Property and Casualty business only.