Schedule I Summary of Investments - Other Than Investments in Affiliates (Details) $ in Millions	Dec. 31, 2017 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 43,759
Amount at which shown on Balance Sheet	45,146
U.S. government and government agencies and authorities (guaranteed and sponsored)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	4,492
Fair Value	4,536
Amount at which shown on Balance Sheet	4,536
States, municipalities and political subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	11,743
Fair Value	12,485
Amount at which shown on Balance Sheet	12,485
Foreign government
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,071
Fair Value	1,110
Amount at which shown on Balance Sheet	1,110
Public utilities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,097
Fair Value	2,188
Amount at which shown on Balance Sheet	2,188
All other corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	10,273
Fair Value	10,616
Amount at which shown on Balance Sheet	10,616
All other mortgage-backed and asset-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	5,936
Fair Value	6,029
Amount at which shown on Balance Sheet	6,029
Total fixed maturities, available-for-sale
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	35,612
Fair Value	36,964
Amount at which shown on Balance Sheet	36,964
Fixed maturities, at fair value using fair value option
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	40
Fair Value	41
Amount at which shown on Balance Sheet	41
Total fixed maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	35,652
Fair Value	37,005
Amount at which shown on Balance Sheet	37,005
Industrial, miscellaneous and all other
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	859
Fair Value	959
Amount at which shown on Balance Sheet	959
Non-redeemable preferred stocks
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	48
Fair Value	53
Amount at which shown on Balance Sheet	53
Equity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	907
Fair Value	1,012
Amount at which shown on Balance Sheet	1,012
Mortgage loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3,175
Fair Value	3,220
Amount at which shown on Balance Sheet	3,175
Futures, options and miscellaneous
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	167
Fair Value	96
Amount at which shown on Balance Sheet	96
Short-term investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,270
Fair Value	2,270
Amount at which shown on Balance Sheet	2,270
Investments in partnerships and trusts
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,588
Amount at which shown on Balance Sheet	$ 1,588