Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		48 Months Ended
	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2010
Catastrophe losses net of reinsurance	$ 16		$ 14	$ 137	$ 103
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	$ 0	200	$ 150	$ 900
Cumulative amounts ceded under AEP Loss Portfolio Transfer	2,800			2,800			$ 2,600
Deferred reinsurance benefit yet to be recognized	347			347			241
Fair value of collateral trust account established by NICO under AEP Loss Portfolio Transfer	$ 2,500			$ 2,500			$ 2,800