Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2010
Catastrophe losses net of reinsurance		$ 165	$ 141	$ 156
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development Life & Group		(45)	(50)	(14)
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Workers' Compensation Reinsurance Pool				26
Loss Portfolio Transfer [Member]
Net A&EP claim and allocated claim adjustment expense reserves ceded to NICO					$ 1,600
Aggregate limit under A&EP Loss Portfolio Transfer					4,000
A&EP claim and allocated claim adjustment expense reserves ceded under existing third party reinsurance contracts transferred to NICO under A&EP Loss Portfolio Transfer					1,200
Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer					2,000
Net reinsurance receivables transferrred to NICO under A&EP Loss Portfolio Transfer					215
Total consideration paid to NICO under AEP Loss Portfolio Transfer					$ 2,200
Net AEP adverse development before consideration of LPT	$ 200	200	150	$ 0
Unfavorable Development related to Asbestos Claims and Higher than Expected Severity in Pollution Claims			150
Cumulative amounts ceded under AEP Loss Portfolio Transfer		2,800	2,600
Deferred reinsurance benefit yet to be recognized		334	$ 241
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer		$ 2,800