Investments - Composition of US and Foreign Corporate Securities (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Collateralized Mortgage Backed Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Percent Collateralized Mortgage Backed Securities including Interest-Only Strip or Principal-Only Strip	48.70%	52.50%
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
Amortized Cost	$ 1,026	$ 1,139
Gross Unrealized Capital Gains	153	55
Gross Unrealized Capital Losses	0	21
Fair Value	1,179	1,173
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
Amortized Cost	2,614	2,707
Gross Unrealized Capital Gains	281	101
Gross Unrealized Capital Losses	1	47
Fair Value	2,894	2,761
Industrial and other companies
Available-for-sale Securities Including Securities Pledged [Line Items]
Amortized Cost	7,150	7,604
Gross Unrealized Capital Gains	680	152
Gross Unrealized Capital Losses	21	214
Fair Value	7,809	7,542
Energy
Available-for-sale Securities Including Securities Pledged [Line Items]
Amortized Cost	1,710	1,884
Gross Unrealized Capital Gains	191	55
Gross Unrealized Capital Losses	36	81
Fair Value	1,865	1,858
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
Amortized Cost	2,888	2,974
Gross Unrealized Capital Gains	304	80
Gross Unrealized Capital Losses	5	74
Fair Value	3,187	2,980
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
Amortized Cost	846	729
Gross Unrealized Capital Gains	78	14
Gross Unrealized Capital Losses	2	17
Fair Value	922	726
U.S. and Foreign Corporate Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Amortized Cost	16,234	17,037
Gross Unrealized Capital Gains	1,687	457
Gross Unrealized Capital Losses	65	454
Fair Value	$ 17,856	$ 17,040