Summary of Significant Accounting Policies - Securities loaned and Recognition of premium revenues and contract charges, and related benefits and interest credited (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Securities loaned
Cash collateral received as percentage of fair value of domestic securities loaned	102.00%
Cash collateral received as a percent of fair value of foreign securities loaned	105.00%
Deferred Policy Acquisition and Sales Inducement Costs
Unearned premiums	$ 15,343	$ 14,510
Valuation allowance for uncollectible premium installment receivables	90	77
Present value of future profits	39	45
Amortization expense of present value of future profits	6	$ 2	$ 6
Allstate Protection
Deferred Policy Acquisition and Sales Inducement Costs
Unearned premiums	12,570
Service Businesses
Deferred Policy Acquisition and Sales Inducement Costs
Unearned premiums	$ 2,760
Minimum
Deferred Policy Acquisition and Sales Inducement Costs
Period for amortization of DAC and DSI for interest-sensitive life, fixed annuities and other investment contracts	15 years
Maximum
Securities loaned
Securities lending transactions length	30 days
Deferred Policy Acquisition and Sales Inducement Costs
Period for amortization of DAC and DSI for interest-sensitive life, fixed annuities and other investment contracts	30 years
Personal Lines [Member] | Minimum
Deferred Policy Acquisition and Sales Inducement Costs
Property and casualty insurance contract, terms	6 months
Period for amortization of DAC for property-liability insurance	6 months
Personal Lines [Member] | Maximum
Deferred Policy Acquisition and Sales Inducement Costs
Property and casualty insurance contract, terms	12 months
Period for amortization of DAC for property-liability insurance	12 months
Interest-sensitive life insurance contract charges | Minimum
Deferred Policy Acquisition and Sales Inducement Costs
Period for amortization of DAC and DSI for interest-sensitive life, fixed annuities and other investment contracts	10 years
Interest-sensitive life insurance contract charges | Maximum
Deferred Policy Acquisition and Sales Inducement Costs
Period for amortization of DAC and DSI for interest-sensitive life, fixed annuities and other investment contracts	20 years
Protection Plans and Other Contracts [Member] | Minimum
Deferred Policy Acquisition and Sales Inducement Costs
Period for amortization of DAC for property-liability insurance	1 year
Protection Plans and Other Contracts [Member] | Maximum
Deferred Policy Acquisition and Sales Inducement Costs
Period for amortization of DAC for property-liability insurance	5 years