Reserve for Unpaid Losses and Loss Adjustment Expenses - PC Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended		12 Months Ended	16 Months Ended	45 Months Ended
	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2016	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance Recoverables, Gross		$ 5,777			$ 5,777	$ 5,777	$ 5,641
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					204
Reinsurance Recoverables, Gross		204			204	204
Prior accident year development [1]					304
Change in Deferred Gain on Retroactive Reinsurance		113			113	113
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		96
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
General Liability [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior accident year development [1]		112	$ 62
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						860
Reinsurance Retention Policy, Amount Retained				$ 1,700
Policyholder Benefits and Claims Incurred, Assumed						640
Asbestos and Environmental | Remaining [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Maximum adverse loss development to be ceded under agreement		860
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
PG&E [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Former Gain Contingency, Recognized in Current Period		289			$ 289	$ 289
Abuse Claims [Member] | General Liability [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior accident year development [1]		$ 129