Investments - Composition of US and Foreign Corporate Securities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	$ 1,064	$ 1,139
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	94	55
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	5	21
Fixed maturities, including securities pledged, Fair Value	1,153	1,173
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,695	2,707
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	174	101
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	12	47
Fixed maturities, including securities pledged, Fair Value	2,857	2,761
Industrial and other companies
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	7,404	7,604
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	327	152
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	54	214
Fixed maturities, including securities pledged, Fair Value	7,677	7,542
Energy
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	1,808	1,884
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	116	55
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	42	81
Fixed maturities, including securities pledged, Fair Value	1,882	1,858
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	2,914	2,974
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	156	80
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	22	74
Fixed maturities, including securities pledged, Fair Value	3,048	2,980
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	810	729
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	42	14
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	6	17
Fixed maturities, including securities pledged, Fair Value	846	726
U.S. and Foreign Corporate Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost	16,695	17,037
U.S. and foreign corporate securities, Gross Unrealized Capital Gains	909	457
U.S. and foreign corporate securities, Gross Unrealized Capital Losses	141	454
Fixed maturities, including securities pledged, Fair Value	$ 17,463	$ 17,040