Losses and Loss Expenses - Reconciliation (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities		$ 20,052,443
Unallocated loss expenses		482,680
Discount	[1]	(413,522)
Total allowance recorded relating to reinsurance recoverables		42,119	$ 58,200
Other		295,551
Total net liability for unpaid losses and loss expenses		20,459,271	20,190,839	$ 15,941,715	$ 17,066,330
Unpaid losses and loss expenses recoverable		5,491,297	5,262,706
Unpaid losses and loss expenses		25,939,571	25,439,744	19,353,243	20,481,065
Other
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss expenses recoverable		71,605
Property and casualty operations
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss expenses recoverable	[2]	5,480,300	$ 5,248,905	$ 3,411,528	$ 3,414,735
Insurance Segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities		13,343,138
Insurance Segment | Professional lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities		3,951,383
Unpaid losses and loss expenses recoverable		1,195,720
Insurance Segment | Casualty and other
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities		6,354,077
Unpaid losses and loss expenses recoverable		2,836,800
Insurance Segment | EPC
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities		1,235,716
Unpaid losses and loss expenses recoverable		486,595
Insurance Segment | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities		1,801,962
Unpaid losses and loss expenses recoverable		524,545
Reinsurance Segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities		6,709,305
Reinsurance Segment | Property and other short-tail lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities		2,201,173
Unpaid losses and loss expenses recoverable		242,845
Reinsurance Segment | Casualty and Other Long-Tail Lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net outstanding liabilities		4,508,132
Unpaid losses and loss expenses recoverable		$ 122,190

[1] As noted in Note 10(c), "Losses and Loss Expenses - Loss Reserve Discounting," the Company discounts only certain workers’ compensation (including long term disability) liabilities and certain bodily injury liability claims, emanating from U.K. exposures, predominantly from the U.K. motor liability portfolio.

[2] Property and Casualty business only.