General - Reportable Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	$ 1,261	$ 977
Property-Liability
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	1,182	990
Service Businesses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	(6)	(22)
Underwriting income or Adjusted net income (loss)	11
Allstate Life
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	67	67
Underwriting income or Adjusted net income (loss)	73
Allstate Benefits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	34	27
Underwriting income or Adjusted net income (loss)	31
Allstate Annuities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	97	17
Underwriting income or Adjusted net income (loss)	(25)
Corporate and Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	(113)	(102)
Underwriting income or Adjusted net income (loss)	(103)
Previously reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	1,256	946
Impact of change | Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income (loss) applicable to common shareholders	$ 5	$ 31