Merger Agreement with Allied World Assurance Company Holdings, AG (Allied World) (Details) (Allied World Assurance Company Holdings, AG, USD $) In Millions, except Share data	1 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2011	Jun. 30, 2011	Jun. 12, 2011
Allied World Assurance Company Holdings, AG
Merger Agreement with Allied World Assurance Company Holdings, AG ("Allied World")
Number of common shares of Allied World Assurance Company Holdings, AG that will be received for each common share owned by stockholders of Transatlantic Holdings, Inc. (in shares)				0.88
Amount of merger expenses entity expects to incur	$ 30
Amount of merger expenses which is contingent on the successful closing of the merger	22
Cost incurred related to merger included in other expenses, net		6	6
Termination fee if agreement is terminated by the board of directors of either party to the merger	115
Termination fee if agreement is terminated by stockholders of either party to the merger	35
Maximum reimbursement of merger expenses if agreement is terminated by stockholders of either party to the merger	35
Maximum reimbursement of other parties' expenses if agreement is terminated due to breach of agreement	$ 35