Reserve for Unpaid Losses and Loss Adjustment Expenses - P&C Additional Information (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Catastrophes
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior accident year development	$ (47)	$ (12)
Scenario, Adjustment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	90
Scenario, Adjustment [Member] | Catastrophes
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior accident year development	133
Scenario, Plan [Member] | Catastrophes
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	$ 850