Investments (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Fair value	$ 40,241,000,000	$ 38,017,000,000
Cost or Amortized Cost	37,633,000,000	36,849,000,000
Gross Unrealized Losses	541,000,000	804,000,000
Net unrealized gains (losses) related to changes in fair value of trading securities still held included in net investment income			(5,000,000)
Number of non-income producing fixed maturity securities	9	7
Fair value of non income fixed maturity security	3,000,000	3,000,000
Number of investments that exceed ten percent of stockholders' equity
Net unrealized gains (losses) related to changes in the fair value of securities for which the fair value option has been elected	2,000,000	(1,000,000)
Limited partnership investments	2,245,000,000	2,309,000,000
Undistributed earnings of limited partnership investments	560,000,000	723,000,000
Percentage of limited partnerships reported on a current basis	58.00%
Percentage of limited partnerships reported on a one month lag	25.00%
Number of active limited partnership investments held	79	75
Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments	81.00%	85.00%
Percentage of equity related limited partnership hedge fund strategies	46.00%
Percentage of multistrategy approach limited partnership hedge fund strategies	32.00%
Percentage of distressed investments limited partnership hedge fund strategies	19.00%
Percentage of fixed income limited partnership hedge fund strategies	3.00%
Limited partnerships invested in private equity	14.00%	11.00%
Carrying value of ten largest limited partnerships	1,218,000,000	1,321,000,000
Carrying value of limited partnerships as percentage of aggregate partnership equity	4.00%
Income as percentage of change in partnership equity for all limited partnerships	4.00%	3.00%	4.00%
Commitments to purchase various privately placed debt securities	95,000,000
Commitments to sell various privately placed debt securities	69,000,000
Obligations on Unfunded Bank Loan Participations	6,000,000
Mortgage loan commitments	48,000,000
Carrying value of securities deposited under requirements of regulatory authorities	3,500,000,000	2,900,000,000
Cash and securities deposited as collateral for letters of credit	5,000,000	6,000,000
Carrying value of assets deposited to secure reinsurance and third party obligations	288,000,000	298,000,000
Corporate and other bonds, financial industry sector [Member]
Fair value	1,682,000,000
Cost or Amortized Cost	1,788,000,000
States, municipalities and political subdivisions [Member]
Fair value	9,782,000,000	7,889,000,000
Cost or Amortized Cost	9,018,000,000	8,157,000,000
Gross Unrealized Losses	136,000,000	410,000,000
Percentage of gross unrealized losses by AA or higher securities	83.00%
Gross unrealized losses of largest exposure, Puerto Rico sales tax revenue bonds	80,000,000
Asset-backed [Member]
Fair value	8,084,000,000	7,846,000,000
Cost or Amortized Cost	8,097,000,000	8,001,000,000
Gross Unrealized Losses	246,000,000	314,000,000
Number of different asset backed securities comprising total fair value of asset backed securities	2,010
Number of securities with sub-prime and alt-a exposure	112
Mortgage backed securities, issued by U.S. Government sponsored enterprises [Member]
Gross Unrealized Losses	35,000,000
Non-agency structured residential mortgage backed securities [Member]
Gross Unrealized Losses	148,000,000
Number of securities with at least one trade lot in a gross unrealized loss position	131
Severity of gross unrealized loss as percentage of amortized cost	11.00%
Commercial mortgage-backed [Member]
Fair value	1,354,000,000	993,000,000
Cost or Amortized Cost	1,365,000,000	994,000,000
Gross Unrealized Losses	59,000,000	41,000,000
Number of securities with at least one trade lot in a gross unrealized loss position	61
Severity of gross unrealized loss as percentage of amortized cost	9.00%
Other asset-backed [Member]
Fair value	955,000,000	763,000,000
Cost or Amortized Cost	946,000,000	753,000,000
Gross Unrealized Losses	4,000,000	8,000,000
Number of securities with at least one trade lot in a gross unrealized loss position	51
Severity of gross unrealized loss as percentage of amortized cost	1.00%
Mortgage loans [Member]
Percentage of the carrying value of commercial mortgage loans related to credit tenant loans	14.00%	40.00%
Commercial mortgage loans past due or in non-accrual status
Valuation allowances recorded on commercial mortgage loans
Investment commitment [Member]
Future capital call commitments	$ 129,000,000