Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - Asbestos and Environmental Reserves - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	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	$ 125	$ 150	$ 178
Provision for uncollectible third-party reinsurance, released	25	25	$ 16
Cumulative amounts ceded under AEP Loss Portfolio Transfer	3,300	3,200
Deferred reinsurance benefit yet to be recognized	398	392
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer	$ 4,200	$ 3,700