Reserve for Unpaid Losses and Loss Adjustment Expenses - PC Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	22 Months Ended	45 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Jun. 30, 2017	Mar. 31, 2021	Sep. 30, 2020	Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense_BSASettlement	$ 650
Reinsurance Recoverables, Gross	6,181			$ 6,181		$ 6,119
Adverse Development Cover Navigators Group [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Payments for Reinsurance		$ 91
Policyholder Benefits and Claims Incurred, Assumed				215
Change in Deferred Gain on Retroactive Reinsurance	124			124
Reinsurance Recoverables, Gross	215			215
Prior accident year development [1]				315
Increase (Decrease) in Deferred Revenue	6	$ 29
Adverse Development Cover Navigators Group [Member] | Remaining [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Maximum adverse loss development to be ceded under agreement	85
Adverse Development Cover Navigators Group [Member] | Retention Layer for Reserve for the Covered Liabilities as of the Inception Date [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance Retention Policy, Amount Retained	1,816
Adverse Development Cover Navigators Group [Member] | Retention Layer Above Reserve for the Covered Liabilities as of the Inception Date [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance Retention Policy, Amount Retained	100
Asbestos and Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Payments for Reinsurance			$ 650
Maximum adverse loss development to be ceded under agreement					$ 640
Reinsurance Retention Policy, Amount Retained			$ 1,700
Policyholder Benefits and Claims Incurred, Assumed					$ 860
Asbestos and Environmental | Asbestos and Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Policyholder Benefits and Claims Incurred, Assumed	860
Change in Deferred Gain on Retroactive Reinsurance	210			$ 210
Asbestos and Environmental | Remaining [Member] | Asbestos and Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Maximum adverse loss development to be ceded under agreement	640
Maximum | Adverse Development Cover Navigators Group [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Maximum adverse loss development to be ceded under agreement	300
Maximum | Asbestos and Environmental
Liability for Claims and Claims Adjustment Expense [Line Items]
Maximum adverse loss development to be ceded under agreement	$ 1,500