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	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	$ (52)	$ (48)
Reinsurance Recoverable, Credit Loss Expense (Reversal)	(1)	(3)
Reinsurance Recoverable, Allowance for Credit Loss, Recovery	(1)	(1)
Ending allowance for uncollectible reinsurance	(54)	(54)
Operating Segments [Member] \| Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	(49)	(48)
Reinsurance Recoverable, Credit Loss Expense (Reversal)	(1)	(2)
Reinsurance Recoverable, Allowance for Credit Loss, Recovery	(1)	(1)
Ending allowance for uncollectible reinsurance	(51)	(51)
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	(117)	(114)
Ending allowance for uncollectible reinsurance	(111)	(111)
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	(114)	(114)
Ending allowance for uncollectible reinsurance	(108)	(108)
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	(65)	(66)
Ending allowance for uncollectible reinsurance	(57)	(57)
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	(65)	(66)
Ending allowance for uncollectible reinsurance	(57)	(57)
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	(52)	(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	(49)	(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)