Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2010
Liability for Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses net of reinsurance	$ 38	$ 26	$ 96	$ 60
Asbestos and Environmental Reserves
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 transferrred 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	0	$ 0	0	$ 113
Cumulative amounts ceded under AEP Loss Portfolio Transfer	3,100		3,100		$ 3,100
Deferred reinsurance benefit yet to be recognized	338		338		374
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer	$ 3,100		$ 3,100		$ 2,700