Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2016
Asbestos
Loss Contingencies [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts	$ 910
Environmental
Loss Contingencies [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts	133
Asbestos and Environmental
Loss Contingencies [Line Items]
Reinsurance premium		$ 650
Maximum adverse loss development to be ceded under agreement	977
Reinsurance Retention Policy, Amount Retained		$ 1,700
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	523
Asbestos and Environmental | Maximum
Loss Contingencies [Line Items]
Maximum adverse loss development to be ceded under agreement	$ 1,500