Reserve for Unpaid Losses and Loss Adjustment Expenses - PC Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		10 Months Ended	12 Months Ended	39 Months Ended
	Mar. 31, 2020	Mar. 31, 2018	Mar. 31, 2020	Dec. 31, 2016	Mar. 31, 2020
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	136		$ 236
Change in Deferred Gain on Retroactive Reinsurance	45		$ 45		$ 45
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	164
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	100	$ 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	860
Policyholder Benefits and Claims Incurred, Assumed	640				$ 640
Reinsurance Retention Policy, Amount Retained				$ 1,700
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