Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2010
Liability for Claims and Claims Adjustment Expense [Line Items]
Retroactive reinsurance benefit recognized	$ 14	$ 22
Fair value of collateral trust account represented as percentage of gross loss portfolio transfer reserves	150.00%
Unusual or Infrequent Item, or Both, Net of Insurance Proceeds	$ 75	$ 58
Asbestos and Environmental Reserves
Liability for Claims and Claims Adjustment Expense [Line Items]
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 transferred to NICO under AEP Loss Portfolio Transfer				215
Total consideration paid to NICO under AEP Loss Portfolio Transfer				$ 2,200
Cumulative amounts ceded under AEP Loss Portfolio Transfer	3,200		$ 3,200
Deferred reinsurance benefit yet to be recognized	378		392
Fair value of collateral trust account established by NICO under AEP Loss Portfolio Transfer	2,700		$ 3,700
COVID-19
Liability for Claims and Claims Adjustment Expense [Line Items]
Unusual or Infrequent Item, or Both, Net of Insurance Proceeds	13
Weather Related Events
Liability for Claims and Claims Adjustment Expense [Line Items]
Unusual or Infrequent Item, or Both, Net of Insurance Proceeds	$ 62