Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Jan. 01, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Net of Tax			$ 102			$ 102
Net unrealized investment gains (losses):
Net unrealized investment gains		$ 251		$ 0		585	$ 0
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax [Abstract]
Other comprehensive income (loss)		251	331	(31)	$ 344	582	313
Foreign currency translation adjustments		3		(27)		4	(26)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax [Abstract]
Adoption of new accounting standard	[1]							$ (13)
Net cash flow hedges		(3)		(4)		(7)	339
Pension and OPEB plans:
Adoption of new accounting standard	[1]							(13)
Other comprehensive income (loss)		251	331	(31)	344	582	313
Adoption of new accounting standard	[1]							(13)
Other comprehensive income (loss)		251	331	(31)	344	582	313
Accumulated Other Comprehensive Income (Loss), Net of Tax		684				684
Cash flow hedge gain reclassified during next 12 months		18				18
Net Unrealized Gains (Losses) Investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Net of Tax		431	97	0	0	97	0
Net unrealized investment gains (losses):
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax		259		0		607	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax		(8)		0		(22)	0
Pension and OPEB plans:
Accumulated Other Comprehensive Income (Loss), Net of Tax		682	431	0	0	682	0
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, before Tax		309		0		719	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax		(11)		0		(30)	0
Accumulated Foreign Currency Adjustment Attributable to Parent
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Net of Tax		(157)	(158)	(128)	(129)	(158)	(129)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax [Abstract]
Other comprehensive income (loss)		3		(27)		4	(26)
Pension and OPEB plans:
Other comprehensive income (loss)		3		(27)		4	(26)
Other comprehensive income (loss)		3		(27)		4	(26)
Accumulated Other Comprehensive Income (Loss), Net of Tax		(154)	(157)	(155)	(128)	(154)	(155)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Net of Tax		308	312	325	(15)	312	(15)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax [Abstract]
Adoption of new accounting standard								(3)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		(3)		0		0	344
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		0		(4)		(7)	(5)
Pension and OPEB plans:
Adoption of new accounting standard								(3)
Adoption of new accounting standard								(3)
Accumulated Other Comprehensive Income (Loss), Net of Tax		305	308	321	325	305	321
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, before Tax		(4)		0		0	464
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax		0		(6)		(9)	(7)
Pension and OPEB Plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Net of Tax		(149)	(149)	(25)	(21)	(149)	(21)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax [Abstract]
Adoption of new accounting standard								(4)
Pension and OPEB plans:
Adoption of new accounting standard								(4)
Adoption of new accounting standard								(4)
Accumulated Other Comprehensive Income (Loss), Net of Tax		(149)	(149)	(25)	(25)	(149)	(25)
Accumulated Other Comprehensive Income (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Net of Tax		433	102	172	(165)	102	(165)
Pension and OPEB plans:
Accumulated Other Comprehensive Income (Loss), Net of Tax		684	433	141	172	684	141
Accumulated Other Comprehensive Income (Loss)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax [Abstract]
Other comprehensive income (loss)		251	331	(31)	344	582	313
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax [Abstract]
Adoption of new accounting standard	[1]							(7)
Pension and OPEB plans:
Adoption of new accounting standard	[1]							(7)
Other comprehensive income (loss)		251	331	(31)	344	582	313
Adoption of new accounting standard	[1]							$ (7)
Other comprehensive income (loss)		$ 251	$ 331	$ (31)	$ 344	$ 582	$ 313

[1] Reflects the adoption of Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers, which resulted in a reduction to retained earnings of $13 million and the adoption of ASU 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income, which resulted in a reduction to accumulated other comprehensive income and an increase to retained earnings of $7 million.