Subsequent Event (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Feb. 05, 2021	Dec. 31, 2020	Dec. 31, 2019
Subsequent Event [Abstract]
Reinsurance recoverable			$ 4,457	$ 4,179
CNA Financial [Member] | Plan [Member]
Subsequent Event [Abstract]
Loss contingency, after tax and noncontrolling interests	$ 11
Subsequent Event [Member] | CNA Financial [Member] | Loss Portfolio Transfer for Workers' Compensation Liabilities [Member]
Subsequent Event [Abstract]
Reinsurance recoverable		$ 690
Subsequent Event [Member] | CNA Financial [Member] | Loss Portfolio Transfer for Workers' Compensation Liabilities [Member] | Maximum [Member]
Subsequent Event [Abstract]
Reinsurance recoverable		$ 1,000