Investments (details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Investments disclosure details
Fixed maturities, due in one year or less, amortized cost	$ 5,829,000,000
Fixed maturities, due after 1 year through 5 years, amortized cost	19,843,000,000
Fixed maturities, due after 5 years through 10 years, amortized cost	17,107,000,000
Fixed maturities, due after 10 years, amortized cost	13,927,000,000
Fixed maturities excluding mortgage-backed securities, collateralized mortgage obligations and pass-through securities, amortized cost	56,706,000,000
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities, amortized cost	3,288,000,000
Fixed maturities, amortized cost	59,994,000,000	60,170,000,000
Fixed maturities, due in one year or less, fair value	5,901,000,000
Fixed maturities, due after 1 year through 5 years, fair value	21,144,000,000
Fixed maturities, due after 5 years through 10 years, fair value	18,694,000,000
Fixed maturities, due after 10 years, fair value	14,978,000,000
Fixed maturities excluding mortgage-backed securities, collateralized mortgage obligations and pass-through securities, fair value	60,717,000,000
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities, fair value	3,515,000,000
Fixed maturities, at fair value	64,232,000,000	62,820,000,000
Securities on loan as part of a tri-party lending agreement	126,000,000	186,000,000
Proceeds from sales of fixed maturities classified as available for sale	1,161,000,000	3,713,000,000	2,805,000,000
Insurance subsidiaries' securities on deposit at financial institutions in certain states pursuant to the respective states' insurance regulatory requirements	4,700,000,000	4,510,000,000
Fair value of funds deposited with third parties to be used as collateral to secure various liabilities on behalf of insureds, cedants and other creditors	90,000,000	86,000,000
Fair value of other investments pledged as collateral securing outstanding letters of credit	59,000,000	88,000,000
Proceeds from sales of equity securities	135,000,000	201,000,000	65,000,000
Proceeds from sale of investment in Verisk, equity securities		115,000,000
Proceeds from sale of investment in Verisk, other investments		115,000,000	184,000,000
Proceeds from sale of real estate investments		10,000,000
Accumulated depreciation on real estate held for investment purposes	226,000,000	200,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2012	80,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2013	72,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2014	62,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2015	53,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2016	36,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2017 and thereafter	56,000,000
Combined average days to maturity of short-term securities (in days)	79
Amortized cost of short-term securities (which approximates fair value)	3,590,000,000	5,620,000,000
Unrealized investment losses for securities for which fair value is less than 80% of amortized cost, as a percentage of the combined fixed maturity and equity security portfolios on a pretax basis	less than 1%
Unrealized investment losses for securities for which fair value is less than 80% of amortized cost, as a percentage of shareholders' equity on an after-tax basis	less than 1%
Concentration of credit risk	The Company was not exposed to any concentration of credit risk of a single issuer greater than 5% of shareholders' equity of the Company.	The Company was not exposed to any concentration of credit risk of a single issuer greater than 5% of shareholders' equity of the Company.
Below investment grade assets included in fixed maturities	1,960,000,000	1,880,000,000
Gross investment income	2,910,000,000	3,093,000,000	2,809,000,000
Investment expenses	31,000,000	34,000,000	33,000,000
Net investment income	2,879,000,000	3,059,000,000	2,776,000,000
Notional value of the open U.S. Treasury futures contract	900,000,000	400,000,000
Net realized investment gains (losses) related to U.S. Treasury futures contracts	(62,000,000)	(30,000,000)	23,000,000
Net realized investment gains related to Platinum warrants		5,000,000	7,000,000
Number of non-public stock purchase warrants of Platinum Underwriters Holdings, Ltd. owned		$ 6,000,000	$ 6,000,000
Net realized investment gains (losses) related to embedded derivatives	$ (2,000,000)	$ (1,000,000)	$ 1,000,000