Investments - Composition of US and Foreign Corporate Securities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities including securities pledged, Amortized Cost	$ 1,139	$ 1,145
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	55	114
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	21	1
U.S. and foreign corporate securities, Fair Value	1,173	1,258
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities including securities pledged, Amortized Cost	2,707	2,750
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	101	185
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	47	4
U.S. and foreign corporate securities, Fair Value	2,761	2,931
Industrial and other companies
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities including securities pledged, Amortized Cost	7,604	7,953
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	152	532
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	214	65
U.S. and foreign corporate securities, Fair Value	7,542	8,420
Energy
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities including securities pledged, Amortized Cost	1,884	1,970
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	55	159
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	81	33
U.S. and foreign corporate securities, Fair Value	1,858	2,096
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities including securities pledged, Amortized Cost	2,974	2,725
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	80	216
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	74	11
U.S. and foreign corporate securities, Fair Value	2,980	2,930
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities including securities pledged, Amortized Cost	729	697
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	14	52
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	17	2
U.S. and foreign corporate securities, Fair Value	726	747
U.S. and Foreign Corporate Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities including securities pledged, Amortized Cost	17,037	17,240
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	457	1,258
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	454	116
U.S. and foreign corporate securities, Fair Value	$ 17,040	$ 18,382