Investments - Composition of US and Foreign Corporate Securities (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Communications
Available-for-sale Securities Including Securities Pledged [Line Items]
Amortized Cost	$ 1,083	$ 1,139
Gross Unrealized Capital Gains	133	55
Gross Unrealized Capital Losses	0	21
Fair Value	1,216	1,173
Financial
Available-for-sale Securities Including Securities Pledged [Line Items]
Amortized Cost	2,628	2,707
Gross Unrealized Capital Gains	246	101
Gross Unrealized Capital Losses	4	47
Fair Value	2,870	2,761
Industrial and other companies
Available-for-sale Securities Including Securities Pledged [Line Items]
Amortized Cost	7,236	7,604
Gross Unrealized Capital Gains	521	152
Gross Unrealized Capital Losses	26	214
Fair Value	7,731	7,542
Energy
Available-for-sale Securities Including Securities Pledged [Line Items]
Amortized Cost	1,722	1,884
Gross Unrealized Capital Gains	162	55
Gross Unrealized Capital Losses	32	81
Fair Value	1,852	1,858
Utilities
Available-for-sale Securities Including Securities Pledged [Line Items]
Amortized Cost	2,885	2,974
Gross Unrealized Capital Gains	219	80
Gross Unrealized Capital Losses	12	74
Fair Value	3,092	2,980
Transportation
Available-for-sale Securities Including Securities Pledged [Line Items]
Amortized Cost	809	729
Gross Unrealized Capital Gains	64	14
Gross Unrealized Capital Losses	2	17
Fair Value	871	726
U.S. and Foreign Corporate Securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Amortized Cost	16,363	17,037
Gross Unrealized Capital Gains	1,345	457
Gross Unrealized Capital Losses	76	454
Fair Value	$ 17,632	$ 17,040
Collateralized Mortgage Backed Securities [Member]
Available-for-sale Securities Including Securities Pledged [Line Items]
Percent Collateralized Mortgage Backed Securities including Interest-Only Strip or Principal-Only Strip	49.20%	52.50%