Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Name:	us-gaap_GoodwillImpairmentLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Summary of Significant Accounting Policies - Goodwill (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill
Goodwill allocated to segments		$ 2,545	$ 2,530
Impairment of goodwill and purchased intangibles	$ 125	106	0	$ 125
Allstate Protection
Goodwill
Goodwill allocated to segments		810	810
Service Businesses
Goodwill
Goodwill allocated to segments		1,464	1,449
Allstate Life
Goodwill
Goodwill allocated to segments		175	175
Allstate Benefits
Goodwill
Goodwill allocated to segments		$ 96	$ 96