Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended	42 Months Ended
	Jun. 30, 2020	Dec. 31, 2016	Jun. 30, 2020
Asbestos [Member]
Loss Contingencies [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts	$ 815		$ 815
Environmental [Member]
Loss Contingencies [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts	106		106
Asbestos and Environmental
Loss Contingencies [Line Items]
Maximum adverse loss development to be ceded under agreement	860
Reinsurance Retention Policy, Amount Retained		$ 1,700
Reinsurance Retention Policy, Excess Retention, Amount Reinsured			$ 640
Asbestos and Environmental | Maximum
Loss Contingencies [Line Items]
Maximum adverse loss development to be ceded under agreement	$ 1,500