Investments (details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investments disclosure
Fixed maturities, due in one year or less, amortized cost	$ 7,762,000,000
Fixed maturities, due after 1 year through 5 years, amortized cost	18,447,000,000
Fixed maturities, due after 5 years through 10 years, amortized cost	16,815,000,000
Fixed maturities, due after 10 years, amortized cost	15,725,000,000
Fixed maturities excluding mortgage-backed securities, collateralized mortgage obligations and pass-through securities, amortized cost	58,749,000,000
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities, amortized cost	2,052,000,000
Fixed maturities, amortized cost	60,801,000,000	62,196,000,000
Fixed maturities, due in one year or less, fair value	7,859,000,000
Fixed maturities, due after 1 year through 5 years, fair value	19,325,000,000
Fixed maturities, due after 5 years through 10 years, fair value	17,462,000,000
Fixed maturities, due after 10 years, fair value	16,615,000,000
Fixed maturities excluding mortgage-backed securities, collateralized mortgage obligations and pass-through securities, fair value	61,261,000,000
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities, fair value	2,213,000,000
Fixed maturities, at fair value	63,474,000,000	63,956,000,000
Securities on loan as part of a tri-party lending agreement	296,000,000	131,000,000
Proceeds from sales of fixed maturities classified as available for sale	1,049,000,000	1,635,000,000	1,087,000,000
Insurance subsidiaries' securities on deposit at financial institutions in certain states pursuant to the respective states' insurance regulatory requirements	4,780,000,000	4,770,000,000
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	39,000,000	59,000,000
Fair value of other investments pledged as collateral securing outstanding letters of credit	22,000,000	42,000,000
Fair value of owned securities held by an insurance subsidiary pledged into a Lloyd's trust account to support capital requirements for the Company's operations at Lloyd's	151,000,000	181,000,000
Proceeds from sales of equity securities	158,000,000	86,000,000	37,000,000
Proceeds from sale of real estate investments	15,000,000	18,000,000	53,000,000
Real estate held for sale	0	0
Accumulated depreciation on real estate held for investment purposes	290,000,000	264,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2015	85,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2016	72,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2017	56,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2018	44,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2019	34,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2020 and thereafter	64,000,000
Maximum original maturity of short-term securities	1 year
Combined average days to maturity of short-term securities (in days)	57 days
Amortized cost of short-term securities (which approximates fair value)	$ 4,360,000,000	$ 3,880,000,000