Reinsurance - Reinsurance Recoverable, Allowance for Credit Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	$ 55	$ 48
Reinsurance Recoverable, Credit Loss Expense (Reversal)	(13)	2
Reinsurance Recoverable, Allowance for Credit Loss, Ending Balance	42	52
Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	52	48
Reinsurance Recoverable, Credit Loss Expense (Reversal)	(13)	1
Reinsurance Recoverable, Allowance for Credit Loss, Ending Balance	39	49
Operating Segments [Member] | Group Benefits
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	1	0
Reinsurance Recoverable, Credit Loss Expense (Reversal)	0	0
Reinsurance Recoverable, Allowance for Credit Loss, Ending Balance	1	1
Corporate, Non-Segment [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	2	0
Reinsurance Recoverable, Credit Loss Expense (Reversal)	0	1
Reinsurance Recoverable, Allowance for Credit Loss, Ending Balance	2	2
Uncollectible reinsurance
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	108	114
Reinsurance Recoverable, Allowance for Credit Loss, Ending Balance	98	117
Uncollectible reinsurance | Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	105	114
Reinsurance Recoverable, Allowance for Credit Loss, Ending Balance	95	114
Uncollectible reinsurance | Operating Segments [Member] | Group Benefits
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	1	0
Reinsurance Recoverable, Allowance for Credit Loss, Ending Balance	1	1
Uncollectible reinsurance | Corporate, Non-Segment [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	2	0
Reinsurance Recoverable, Allowance for Credit Loss, Ending Balance	2	2
Disputes [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	53	66
Reinsurance Recoverable, Allowance for Credit Loss, Ending Balance	56	65
Disputes [Member] | Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	53	66
Reinsurance Recoverable, Allowance for Credit Loss, Ending Balance	56	65
Disputes [Member] | Operating Segments [Member] | Group Benefits
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	0	0
Reinsurance Recoverable, Allowance for Credit Loss, Ending Balance	0	0
Disputes [Member] | Corporate, Non-Segment [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	0	0
Reinsurance Recoverable, Allowance for Credit Loss, Ending Balance	0	0
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member]
Reinsurance Recoverable, Past Due [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest				$ 2
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member] | Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Past Due [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest				0
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member] | Operating Segments [Member] | Group Benefits
Reinsurance Recoverable, Past Due [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest				1
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member] | Corporate, Non-Segment [Member]
Reinsurance Recoverable, Past Due [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest				$ 1
Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	55	50
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] | Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	52	48
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] | Operating Segments [Member] | Group Benefits
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	1	1
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] | Corporate, Non-Segment [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Reinsurance Recoverable, Allowance for Credit Loss, Beginning Balance	$ 2	$ 1