Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Net of loss reserve discounts	$ 19	$ 17
Decrease in estimates for claims	2	31
Adjustment expense	27	31
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Adjustment expense	27	31
Favorable reserve development net of premium offsets	35	62
Reinsurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable reserve development net of premium offsets	$ 8	$ 31
Reinsurance reserves ogden discount rate			(0.75%)	2.50%