Losses and Loss Expenses - Loss Reserve Rollforward and Components of Net Losses and Loss Expenses (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Unpaid losses and loss expenses at beginning of year		$ 25,439,744	$ 19,353,243	$ 20,481,065
Unpaid losses and loss expenses recoverable, beginning of year		5,262,706
Net unpaid losses and loss expenses at beginning of year		20,190,839	15,941,715	17,066,330
Increase (decrease) in net losses and loss expenses incurred in respect of losses occurring in:
Current year		6,374,377	5,072,830	3,513,465
Prior year development		(301,542)	(306,630)	(255,072)
Total net incurred losses and loss expenses		6,072,835	4,766,200	3,258,393
Loss and loss expense reserves acquired in business combination		101,315	5,439,876	0
Exchange rate effects		(571,198)	(582,300)	(561,673)
Less net losses and loss expenses paid in respect of losses occurring in:
Current year		1,444,251	1,047,277	381,008
Prior years		3,890,269	4,327,375	3,440,327
Total net paid losses		5,334,520	5,374,652	3,821,335
Net unpaid losses and loss expenses at end of year		20,459,271	20,190,839	15,941,715
Unpaid losses and loss expenses recoverable, end of year		5,491,297	5,262,706
Unpaid losses and loss expenses at end of year		25,939,571	25,439,744	19,353,243
Reserve for reported losses and loss expenses		10,343,481	10,293,448
Reserve for losses incurred but not reported		15,596,090	15,146,296
Property and casualty operations
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Unpaid losses and loss expenses recoverable, beginning of year	[1]	5,248,905	3,411,528	3,414,735
Less net losses and loss expenses paid in respect of losses occurring in:
Unpaid losses and loss expenses recoverable, end of year	[1]	$ 5,480,300	$ 5,248,905	$ 3,411,528

[1] Property and Casualty business only.