Related Party Transactions - Related Party Notes (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Related Party Transactions
Notes due to related parties	$ 140	$ 465
Kennett | 6.74% Surplus Note, due 2029
Related Party Transactions
Notes due to related parties	$ 0	25
Notes payable interest rate (as a percent)	6.74%
Kennett | 3.24% Surplus Note, due 2035
Related Party Transactions
Notes due to related parties	$ 0	100
Notes payable interest rate (as a percent)	3.24%
Kennett | 3.48% Surplus Note, due 2036
Related Party Transactions
Notes due to related parties	$ 0	100
Notes payable interest rate (as a percent)	3.48%
Kennett | 5.93% Surplus Note, due 2038
Related Party Transactions
Notes due to related parties	$ 0	50
Notes payable interest rate (as a percent)	5.93%
Kennett | 4.14% Surplus Note, due 2036
Related Party Transactions
Notes due to related parties	$ 0	50
Notes payable interest rate (as a percent)	4.14%
Allstate New Jersey Insurance Company | 3.25% Note to Allstate New Jersey Insurance Company, due 2034
Related Party Transactions
Notes due to related parties	$ 77	77
Notes payable interest rate (as a percent)	3.25%
American Heritage Life Insurance Company | 3.25% Note to American Heritage Life Insurance Company, due 2034
Related Party Transactions
Notes due to related parties	$ 37	37
Notes payable interest rate (as a percent)	3.25%
Allstate Assurance Company | 3.25% Note to Allstate Assurance Company, due 2034
Related Party Transactions
Notes due to related parties	$ 19	19
Notes payable interest rate (as a percent)	3.25%
First Colonial Insurance Company | 3.25% Note to First Colonial Insurance Company, due 2034
Related Party Transactions
Notes due to related parties	$ 7	$ 7
Notes payable interest rate (as a percent)	3.25%