Investments (details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010 item
Investments disclosure
Fixed maturities, due in one year or less, amortized cost	$ 5,634,000,000
Fixed maturities, due after 1 year through 5 years, amortized cost	21,498,000,000
Fixed maturities, due after 5 years through 10 years, amortized cost	16,789,000,000
Fixed maturities, due after 10 years, amortized cost	14,164,000,000
Fixed maturities excluding mortgage-backed securities, collateralized mortgage obligations and pass-through securities, amortized cost	58,085,000,000
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities, amortized cost	2,744,000,000
Fixed maturities, amortized cost	60,829,000,000	59,994,000,000
Fixed maturities, due in one year or less, fair value	5,719,000,000
Fixed maturities, due after 1 year through 5 years, fair value	22,944,000,000
Fixed maturities, due after 5 years through 10 years, fair value	18,329,000,000
Fixed maturities, due after 10 years, fair value	15,404,000,000
Fixed maturities excluding mortgage-backed securities, collateralized mortgage obligations and pass-through securities, fair value	62,396,000,000
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities, fair value	2,997,000,000
Fixed maturities, at fair value	65,393,000,000	64,232,000,000
Securities on loan as part of a tri-party lending agreement	403,000,000	126,000,000
Proceeds from sales of fixed maturities classified as available for sale	1,087,000,000	1,161,000,000	3,713,000,000
Insurance subsidiaries' securities on deposit at financial institutions in certain states pursuant to the respective states' insurance regulatory requirements	4,940,000,000	4,700,000,000
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	68,000,000	90,000,000
Fair value of other investments pledged as collateral securing outstanding letters of credit	56,000,000	59,000,000
Proceeds from sales of equity securities	37,000,000	135,000,000	201,000,000
Proceeds from sale of investment in Verisk, equity securities			115,000,000
Proceeds from sale of investment in Verisk, other investments			115,000,000
Proceeds from sale of real estate investments	53,000,000		10,000,000
Accumulated depreciation on real estate held for investment purposes	255,000,000	237,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2013	76,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2014	69,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2015	59,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2016	43,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2017	29,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2018 and thereafter	49,000,000
Combined average days to maturity of short-term securities (in days)	0 years 0 months 77 days
Amortized cost of short-term securities (which approximates fair value)	3,480,000,000	3,590,000,000
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	Other than U.S. Treasury securities and obligations of U.S. government and government agencies and authorities, the Company was not exposed to any concentration of credit risk of a single issuer greater than 5% of shareholders' equity of the Company.	Other than U.S. Treasury securities and obligations of U.S. government and government agencies and authorities, 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	2,050,000,000	1,960,000,000
Gross investment income	2,925,000,000	2,910,000,000	3,093,000,000
Investment expenses	36,000,000	31,000,000	34,000,000
Net investment income	2,889,000,000	2,879,000,000	3,059,000,000
Notional value of the open U.S. Treasury futures contracts	800,000,000	900,000,000
Net realized investment losses related to U.S. Treasury futures contracts	14,000,000	62,000,000	30,000,000
Net realized investment gains related to Platinum warrants			5,000,000
Number of non-public stock purchase warrants of Platinum Underwriters Holdings, Ltd. owned			6,000,000
Net realized investment losses related to embedded derivatives		$ 2,000,000	$ 1,000,000
Net realized investment losses related to embedded derivatives	less than $1 million