Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		48 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2010
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (98)		$ (5)	$ (150)	$ (12)
Catastrophe losses net of reinsurance	85		60	121	89
Specialty [Member]
Favorable development in other professional liability and management liability related to lower than expected severity for professional services primarily in accident years 2010 and prior			38		41
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(65)		(13)	(99)	(11)
Unfavorable development in other professional liability and management liability related to lower than expected severity for professional services primarily in accident years 2012 through 2014			37		37
Commercial [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(18)		16	(32)	11
Loss Portfolio Transfer [Member]
Liability for Asbestos and Environmental Claims, Net								$ 1,600
Aggregate limit under AEP Loss Portfolio Transfer								4,000
Claim And Allocated Claim Adjustment Expense Reserves Under Existing Third Party Reinsurance Contracts								1,200
Reinsurance premium paid to NICO under AEP Loss Portfolio Transfer								2,000
Net reinsurance receivables transferrred to NICO under AEP Loss Portfolio Transfer								215
Total consideration paid to NICO under AEP Loss Portfolio Transfer								$ 2,200
Net AEP adverse development before consideration of LPT	0	$ 200	$ 150	200	$ 150	$ 900
Cumulative amounts ceded under AEP Loss Portfolio Transfer	2,800			2,800			$ 2,600
Deferred reinsurance benefit yet to be recognized	359			359			241
Fair value of collateral trust account established by NICO under AEP Loss Portfolio Transfer	$ 2,600			$ 2,600			$ 2,800