Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2010
Liability for Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses net of reinsurance	$ 26	$ 39	$ 60	$ 73
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 loss portfolio transfer	0	$ 0	113	$ 60
Cumulative amounts ceded under AEP loss portfolio transfer	3,000		3,000		$ 2,900
Deferred reinsurance benefit yet to be recognized	351		351		326
Fair value of collateral trust account established by NICO under A&EP loss portfolio transfer	$ 3,000		$ 3,000		$ 3,100