General - Reportable Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	$ 821	$ 678	$ 2,082	$ 1,655
Property-Liability
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	863	630	2,045	1,620
Service Businesses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	(46)	(15)	(52)	(37)
Underwriting income or Adjusted net income (loss)	16	2	27	(1)
Allstate Life
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	67	75	134	142
Underwriting income or Adjusted net income (loss)	68	80	141	151
Allstate Benefits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	39	36	73	63
Underwriting income or Adjusted net income (loss)	37	36	68	65
Allstate Annuities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	88	50	185	67
Underwriting income or Adjusted net income (loss)	52	44	27	79
Corporate and Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	(190)	(98)	(303)	(200)
Underwriting income or Adjusted net income (loss)	(98)	(95)	(201)	(185)
Previously reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	890	637	2,146	1,583
Impact of change | Impact of change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	$ (69)	$ 41	$ (64)	$ 72