Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - Asbestos and Environmental Reserves - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2010
Liability for Claims and Claims Adjustment Expense [Line Items]
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 transferred to NICO under A&EP Loss Portfolio Transfer				215
Total consideration paid to NICO under AEP Loss Portfolio Transfer				$ 2,200
Net A&EP adverse development before consideration of LPT	$ 150	$ 178	$ 60
Provision for uncollectible third-party reinsurance, released	25	16
Cumulative amounts ceded under AEP Loss Portfolio Transfer	3,200	3,100
Deferred reinsurance benefit yet to be recognized	392	374
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer	$ 3,700	$ 2,700