Amount of increase in allowance for credit loss on reinsurance recoverable, from recovery.

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Reinsurance - Reinsurance Recoverable, Allowance for Credit Loss (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	$ (54)	$ (48)
Reinsurance Recoverable, Credit Loss Expense (Reversal)	1	4
Reinsurance Recoverable, Allowance for Credit Loss, Recovery	0	1
Ending allowance for uncollectible reinsurance	(55)	(55)
Operating Segments [Member] \| Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	(51)	(48)
Reinsurance Recoverable, Credit Loss Expense (Reversal)	1	3
Reinsurance Recoverable, Allowance for Credit Loss, Recovery	0	1
Ending allowance for uncollectible reinsurance	(52)	(52)
Operating Segments [Member] \| Group Benefits [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	(1)	0
Reinsurance Recoverable, Credit Loss Expense (Reversal)	0	0
Reinsurance Recoverable, Allowance for Credit Loss, Recovery	0	0
Ending allowance for uncollectible reinsurance	(1)	(1)
Corporate, Non-Segment [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	(2)	0
Reinsurance Recoverable, Credit Loss Expense (Reversal)	0	1
Reinsurance Recoverable, Allowance for Credit Loss, Recovery	0	0
Ending allowance for uncollectible reinsurance	(2)	(2)
Uncollectible reinsurance
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	(111)	(114)
Ending allowance for uncollectible reinsurance	(109)	(109)
Uncollectible reinsurance \| Operating Segments [Member] \| Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	(108)	(114)
Ending allowance for uncollectible reinsurance	(106)	(106)
Uncollectible reinsurance \| Operating Segments [Member] \| Group Benefits [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	(1)	0
Ending allowance for uncollectible reinsurance	(1)	(1)
Uncollectible reinsurance \| Corporate, Non-Segment [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	(2)	0
Ending allowance for uncollectible reinsurance	(2)	(2)
Disputes [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	(57)	(66)
Ending allowance for uncollectible reinsurance	(54)	(54)
Disputes [Member] \| Operating Segments [Member] \| Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	(57)	(66)
Ending allowance for uncollectible reinsurance	(54)	(54)
Disputes [Member] \| Operating Segments [Member] \| Group Benefits [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	0	0
Ending allowance for uncollectible reinsurance	0	0
Disputes [Member] \| Corporate, Non-Segment [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	0	0
Ending allowance for uncollectible reinsurance	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			$ 0	$ 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	0
Cumulative Effect, Period of Adoption, Adjustment [Member] \| Accounting Standards Update 2016-13 [Member] \| Operating Segments [Member] \| Group Benefits [Member]
Reinsurance Recoverable, Past Due [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest			0	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			$ 0	$ 1
Cumulative Effect, Period of Adoption, Adjusted Balance [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	(54)	(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]
Beginning allowance for uncollectible reinsurance	(51)	(48)
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] \| Operating Segments [Member] \| Group Benefits [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	(1)	(1)
Cumulative Effect, Period of Adoption, Adjusted Balance [Member] \| Corporate, Non-Segment [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	$ (2)	$ (1)