The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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Description of the frequency of periodic payments (monthly, quarterly, annual).

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Interest rate stated in the contractual debt agreement.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Amount of debt issuance costs (for example, but not limited to, legal, accounting, broker, and regulatory fees).

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The cash inflow from additional borrowings, net of cash paid to third parties in connection with debt origination.

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Percentage of face value the debt was issued for.

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Senior Notes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified				1 Months Ended					1 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Mar. 06, 2012 Transatlantic Holdings Incorporated	Jun. 26, 2012 2022 Senior Notes	Sep. 20, 2010 2020 Senior Notes	Dec. 14, 2005 2015 Notes Transatlantic Holdings Incorporated	Sep. 30, 2012 2015 Notes Transatlantic Holdings Incorporated	Mar. 06, 2012 2015 Notes Transatlantic Holdings Incorporated	Nov. 23, 2009 2039 Notes Transatlantic Holdings Incorporated	Mar. 06, 2012 2039 Notes Transatlantic Holdings Incorporated
Debt Instrument [Line Items]
Senior notes, face value				$ 400.0	$ 300.0	$ 750.0	$ 667.0		$ 350.0
Senior notes, interest rate				4.95%	5.625%	5.75%			8.00%
Senior notes, maturity date				Jun. 27, 2022	Sep. 15, 2020	Dec. 14, 2015			Nov. 30, 2039
Senior notes, frequency of interest payment				Semi-annually	Semi-annually	Semi-annually			Semi-annually
Senior notes, issuance rate				99.90%
Proceeds from issuance of Senior notes				396.0
Senior notes, effective yield				5.05%				3.00%		6.90%
Senior notes, underwriting discount, commission and other expenses				3.6
Senior notes, fair value	$ 1,974.2	$ 314.8	$ 1,100.0