Schedule I - Summary of Investments Other than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2017 USD ($)
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	$ 80,176
Amount at which shown in the Balance Sheet	82,803
United States government, government agencies and authorities
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	3,580
Fair value	3,616
Amount at which shown in the Balance Sheet	3,616
States, municipalities and political subdivisions
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	8,053
Fair value	8,328
Amount at which shown in the Balance Sheet	8,328
Foreign governments
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	1,005
Fair value	1,021
Amount at which shown in the Balance Sheet	1,021
Public utilities
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	5,655
Fair value	5,988
Amount at which shown in the Balance Sheet	5,988
All other corporate bonds
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	37,341
Fair value	38,038
Amount at which shown in the Balance Sheet	38,038
Asset-backed securities
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	1,266
Fair value	1,272
Amount at which shown in the Balance Sheet	1,272
Residential mortgage-backed securities
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	480
Fair value	578
Amount at which shown in the Balance Sheet	578
Commercial mortgage-backed securities
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	124
Fair value	128
Amount at which shown in the Balance Sheet	128
Redeemable preferred stock
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	21
Fair value	23
Amount at which shown in the Balance Sheet	23
Fixed income securities
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	57,525
Fair value	58,992
Amount at which shown in the Balance Sheet	58,992
Public utilities
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	84
Fair value	99
Amount at which shown in the Balance Sheet	99
Banks, trusts and insurance companies
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	565
Fair value	725
Amount at which shown in the Balance Sheet	725
Industrial, miscellaneous and all other
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	4,591
Fair value	5,506
Amount at which shown in the Balance Sheet	5,506
Nonredeemable preferred stocks
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	221
Fair value	291
Amount at which shown in the Balance Sheet	291
Equity securities
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	5,461
Fair value	6,621
Amount at which shown in the Balance Sheet	6,621
Mortgage loans on real estate
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	4,534
Fair value	4,732
Amount at which shown in the Balance Sheet	4,534
Real estate
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	468
Amount at which shown in the Balance Sheet	468
Real estate acquired in satisfaction of debt
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	0
Amount at which shown in the Balance Sheet	0
Policy loans
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	905
Amount at which shown in the Balance Sheet	905
Derivative instruments
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	127
Fair value	127
Amount at which shown in the Balance Sheet	127
Limited partnership interests
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	6,740
Amount at which shown in the Balance Sheet	6,740
Other long-term investments
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	2,472
Amount at which shown in the Balance Sheet	2,472
Short-term investments
Summary of Investments Other Than Investments in Related Parties
Cost/amortized cost	1,944
Fair value	1,944
Amount at which shown in the Balance Sheet	$ 1,944