Schedule I - Summary of Investments (Details) $ in Millions	Dec. 31, 2017 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 61,583
Fair Value	66,157
Amount Shown on Consolidated Balance Sheet	66,087
U.S. Treasuries
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,047
Fair Value	2,522
Amount Shown on Consolidated Balance Sheet	2,522
U.S. government agencies and authorities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	223
Fair Value	275
Amount Shown on Consolidated Balance Sheet	275
State, municipalities and political subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,856
Fair Value	1,913
Amount Shown on Consolidated Balance Sheet	1,913
U.S. corporate public securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	20,857
Fair Value	23,258
Amount Shown on Consolidated Balance Sheet	23,258
U.S. corporate private securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	5,628
Fair Value	5,833
Amount Shown on Consolidated Balance Sheet	5,833
Foreign corporate public securities and foreign governments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	5,241
Fair Value	5,716
Amount Shown on Consolidated Balance Sheet	5,716
Foreign corporate private securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	4,974
Fair Value	5,161
Amount Shown on Consolidated Balance Sheet	5,161
Residential mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	4,247
Fair Value	4,524
Amount Shown on Consolidated Balance Sheet	4,524
Commercial mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,646
Fair Value	2,704
Amount Shown on Consolidated Balance Sheet	2,704
Other asset-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,488
Fair Value	1,528
Amount Shown on Consolidated Balance Sheet	1,528
Fixed maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	49,207
Fair Value	53,434
Amount Shown on Consolidated Balance Sheet	53,434
Equity securities, available-for-sale
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	353
Fair Value	380
Amount Shown on Consolidated Balance Sheet	380
Short-term investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	471
Fair Value	471
Amount Shown on Consolidated Balance Sheet	471
Mortgage loans on real estate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	8,686
Fair Value	8,748
Amount Shown on Consolidated Balance Sheet	8,686
Policy loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,888
Fair Value	1,888
Amount Shown on Consolidated Balance Sheet	1,888
Limited partnerships/corporations
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	784
Fair Value	784
Amount Shown on Consolidated Balance Sheet	784
Derivatives
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	147
Fair Value	397
Amount Shown on Consolidated Balance Sheet	397
Other investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	47
Fair Value	55
Amount Shown on Consolidated Balance Sheet	$ 47