Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended	45 Months Ended
	Sep. 30, 2020	Dec. 31, 2016	Sep. 30, 2020
Asbestos [Member]
Loss Contingencies [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts	$ 749		$ 749
Environmental [Member]
Loss Contingencies [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts	101		101
Asbestos and Environmental
Loss Contingencies [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured			640
Maximum adverse loss development to be ceded under agreement			$ 860
Reinsurance Retention Policy, Amount Retained		$ 1,700
Asbestos and Environmental | Maximum
Loss Contingencies [Line Items]
Maximum adverse loss development to be ceded under agreement	$ 1,500