Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2010
Liability for Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses net of reinsurance	$ 46	$ 269	$ 106	$ 342
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	339		339		326
Fair value of collateral trust account established by NICO under A&EP loss portfolio transfer	3,200		3,200		$ 3,100
Hurricane Florence
Liability for Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses net of reinsurance	$ 35		$ 35
Hurricane Harvey
Liability for Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses net of reinsurance		149		149
Hurricane Irma
Liability for Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses net of reinsurance		95		95
Hurricane Maria
Liability for Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses net of reinsurance		20		20
Catastrophe
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance premium		$ 6		$ 6