Reinsurance - Reinsurance Recoverable, Allowance for Credit Loss (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Jan. 01, 2020
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	$ (48)
Current period provision	(2)
Ending allowance for uncollectible reinsurance	(52)
Corporate Segment [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	0
Current period provision	(1)
Ending allowance for uncollectible reinsurance	(2)
Operating Segments [Member] \| Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	(48)
Current period provision	(1)
Ending allowance for uncollectible reinsurance	(49)
Operating Segments [Member] \| Group Benefits [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	0
Current period provision	0
Ending allowance for uncollectible reinsurance	(1)
Uncollectible reinsurance
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	(114)
Ending allowance for uncollectible reinsurance	(117)
Uncollectible reinsurance \| Corporate Segment [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	0
Ending allowance for uncollectible reinsurance	(2)
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)
Ending allowance for uncollectible reinsurance	(114)
Uncollectible reinsurance \| Operating Segments [Member] \| Group Benefits [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	0
Ending allowance for uncollectible reinsurance	(1)
Disputes [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	(66)
Ending allowance for uncollectible reinsurance	(65)
Disputes [Member] \| Corporate Segment [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	0
Ending allowance for uncollectible reinsurance	0
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	(66)
Ending allowance for uncollectible reinsurance	(65)
Disputes [Member] \| Operating Segments [Member] \| Group Benefits [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning allowance for uncollectible reinsurance	0
Ending allowance for uncollectible reinsurance	$ 0
Accounting Standards Update 2016-13 [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Cumulative effect of accounting change [1]		$ (2)
Accounting Standards Update 2016-13 [Member] \| Corporate Segment [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Cumulative effect of accounting change [1]		(1)
Accounting Standards Update 2016-13 [Member] \| Operating Segments [Member] \| Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Cumulative effect of accounting change [1]		0
Accounting Standards Update 2016-13 [Member] \| Operating Segments [Member] \| Group Benefits [Member]
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Cumulative effect of accounting change [1]		$ (1)

Amount of cumulative effect of the change in accounting principle or new accounting pronouncement on retained earnings or other components of equity or net assets in the statement of financial position as of the beginning of the earliest period presented.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)(3)
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-Publisher FASB
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-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22583-107794

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- Definition

Amount of increase (decrease) in allowance for credit loss on reinsurance recoverable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919249-210447

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Amount of allowance for credit loss on reinsurance recoverable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919249-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919249-210447

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118955202&loc=SL82895884-210446

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