Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Net of loss reserve discounts	$ 16	$ 8
Increase (decrease) in estimates for claims	13	(19)
Adjustment expense	5	12
Liability recognized for best estimate net of reinsurance	256
Liability recognized for best estimate net of reinsurance, reported losses	219
Liability recognized for best estimate net of reinsurance, incurred but not reported (IBNR)	37
Liability recognized for best estimate net of reinsurance, current accident year losses	46
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability recognized for best estimate net of reinsurance	220
Liability recognized for best estimate net of reinsurance, current accident year losses	43
Favorable reserve development net of premium offsets	8	19
Reinsurance & Monoline Excess
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability recognized for best estimate net of reinsurance	36
Liability recognized for best estimate net of reinsurance, current accident year losses	3
Unfavorable reserve development net of premium offsets	$ 3	$ 7