Investments (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Investment [Line Items]
Gross unrealized losses	$ (626,000,000)	$ (851,000,000)
Carrying value of limited partnerships	2,700,000,000	2,800,000,000
Undistributed earnings of limited partnerships	607,000,000	812,000,000
Percentage of carrying value reported on a current basis	62.70%
Percentage of carrying value reported on one month lag	23.00%
Number of limited partnership investments held	83	84
Percentage of limited partnerships held that employ hedge fund strategies	84.10%	87.40%
Percentage of limited partnerships employing hedge fund strategies focused on equity investments	41.40%
Percentage of limited partnerships employing hedge fund strategies with a multi-strategy approach	39.50%
Future capital call commitments	129,000,000
Percentage of limited partnerships employing hedge fund strategies focused on distressed investments	15.20%
Percentage of limited partnerships employing hedge fund strategies focused on fixed income investments	3.90%
Percentage of limited partnerships invested in private debt and equity	11.70%	9.10%
Commitments to purchase various privately placed debt securities including bank loans	95,000,000
Commitments to sell various privately placed debt securities including bank loans	69,000,000
Aggregate Amortized Cost	38,368,000,000	37,656,000,000
Securities deposited	3,500,000,000	2,900,000,000
Securities deposited as collateral for letters of credit	288,000,000	298,000,000
Mortgage loan commitments	48,000,000
Ownership percentage of aggregate partnership equity	4.10%	4.20%
Changes in ownership percentage of aggregate partnership equity	3.90%	3.50%	4.40%
Cash and securities with carrying values deposited as collateral for letters of credit	5,000,000	6,000,000
Fixed Maturity Securities [Member]
Investment [Line Items]
Number of non-income producing fixed maturity securities held	9	7
Gross unrealized losses	(554,000,000)	(808,000,000)
Aggregate fair value	3,000,000	3,000,000
Aggregate Amortized Cost	37,466,000,000	36,677,000,000
States, Municipalities And Political Subdivisions [Member]
Investment [Line Items]
Gross unrealized losses	(136,000,000)	(410,000,000)
Percentage of securities with gross unrealized losses in this category are rated AA or higher	83.10%
Aggregate Amortized Cost	9,018,000,000	8,157,000,000
Asset-Backed Securities [Member]
Investment [Line Items]
Fair value of total asset-backed holdings	8,100,000,000
Number of different asset backed securities comprising total fair value of asset backed securities	2,011
Gross unrealized losses	(246,000,000)	(314,000,000)
Aggregate Amortized Cost	8,097,000,000	8,002,000,000
Residential Mortgage-Backed [Member]
Investment [Line Items]
Gross unrealized losses	(183,000,000)	(265,000,000)
Severity of unrealized loss as a percentage of amortized cost for residential mortgage-backed securities	10.60%
Aggregate Amortized Cost	5,786,000,000	6,255,000,000
Agency Mortgage-Backed Securities [Member]
Investment [Line Items]
Gross unrealized losses	(35,000,000)
Non-Agency Structured Securities [Member]
Investment [Line Items]
Investment owned, balance, contracts	131
Gross unrealized losses	(148,000,000)
Commercial Mortgage-Backed [Member]
Investment [Line Items]
Investment owned, balance, contracts	61
Gross unrealized losses	(59,000,000)	(41,000,000)
Severity of unrealized loss as a percentage of amortized cost for Commercial Mortgage backed securities	8.90%
Aggregate Amortized Cost	1,365,000,000	994,000,000
Other Asset-Backed [Member]
Investment [Line Items]
Investment owned, balance, contracts	51
Gross unrealized losses	(4,000,000)	(8,000,000)
Severity of unrealized loss as a percentage of amortized cost for states, municipalities and political subdivision securities	1.00%
Aggregate Amortized Cost	946,000,000	753,000,000
Sub-Prime/Alt-A Asset Backed Securities [Member]
Investment [Line Items]
Investment owned, balance, contracts	112
Corporate And Other Bonds [Member]
Investment [Line Items]
Gross unrealized losses	(154,000,000)	(70,000,000)
Aggregate Amortized Cost	19,086,000,000	19,503,000,000
Ten Largest Limited Partnership Holdings [Member]
Investment [Line Items]
Carrying value of limited partnerships	1,300,000,000	1,400,000,000
Obligations on unfunded bank loan participations	6,000,000
Financial Industry Sector Holdings [Member]
Investment [Line Items]
Aggregate fair value	1,700,000,000
Aggregate Amortized Cost	1,800,000,000
PUERTO RICO
Investment [Line Items]
Gross unrealized losses	$ (80,000,000)