Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	45 Months Ended
	Mar. 31, 2021	Jun. 30, 2017	Sep. 30, 2020
Asbestos and Environmental
Loss Contingencies [Line Items]
Maximum adverse loss development to be ceded under agreement			$ 640
Reinsurance Retention Policy, Amount Retained		$ 1,700
Reinsurance Retention Policy, Excess Retention, Amount Reinsured			$ 860
Asbestos Issue
Loss Contingencies [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts	$ 676
Environmental Issue
Loss Contingencies [Line Items]
Liability for unpaid losses and loss adjustment expenses, at undiscounted amounts	79
Asbestos and Environmental | Asbestos and Environmental
Loss Contingencies [Line Items]
Change in Deferred Gain on Retroactive Reinsurance	210
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	860
Maximum | Asbestos and Environmental
Loss Contingencies [Line Items]
Maximum adverse loss development to be ceded under agreement	$ 1,500