Summary of Significant Accounting Policies - Goodwill (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill
Goodwill allocated to segments	$ 2,181	$ 2,181	$ 1,219
Goodwill impairment	125	125	$ 0	$ 0
Allstate Protection
Goodwill
Goodwill allocated to segments	810	810
Service Businesses
Goodwill
Goodwill allocated to segments	1,100	1,100
Allstate Life
Goodwill
Goodwill allocated to segments	175	175
Allstate Benefits
Goodwill
Goodwill allocated to segments	$ 96	$ 96