Capitalization - Changes in Stockholders' Equity (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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 6,932	$ 6,814	$ 7,375	$ 3,925	$ 6,814	$ 3,925
Beginning balance, shares		511,915,450			511,915,450
Net income	312	$ 320	299	1,369	$ 632	1,668
Other comprehensive loss, net of tax	(5)	(5)	(13)	(56)	(10)	(69)
Stock-based compensation	9	7	19	19
Stock Investment Plan and certain share-based benefit plans	21	1	19	6
Cash dividends declared on common stock		(202)		(343)
Cash dividends declared on preferred stock		(3)		(42)
Stock issuance				2,462
Impact of adopting new accounting pronouncements				35
Conversion of Series A Convertible Preferred Stock		0
Ending balance	$ 7,269	6,932	7,699	7,375	$ 7,269	7,699
Ending balance, shares	532,092,829				532,092,829
Amount of beneficial conversion					$ 0	296
Amortization of beneficial conversion	$ 0		148		0	261
Series A Convertible Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 21	$ 71	$ 162	$ 0	$ 71	$ 0
Beginning balance, shares	200,000	700,000	1,600,000	0	700,000	0
Stock issuance				$ 162
Stock issuance, shares				1,600,000
Conversion of Series A Convertible Preferred Stock		$ (50)
Conversion of Series A Convertible Preferred Stock, shares		500,000
Ending balance	$ 21	$ 21	$ 162	$ 162	$ 21	$ 162
Ending balance, shares	200,000	200,000	1,600,000	1,600,000	200,000	1,600,000
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 53	$ 51	$ 48	$ 44	$ 51	$ 44
Beginning balance, shares	531,000,000	512,000,000	477,000,000	445,000,000	512,000,000	445,000,000
Stock Investment Plan and certain share-based benefit plans				$ 1
Stock Investment Plan and certain share-based benefit plans, shares	1,000,000	1,000,000	1,000,000	2,000,000
Stock issuance				$ 3
Stock issuance, shares				30,000,000
Conversion of Series A Convertible Preferred Stock		$ 2
Conversion of Series A Convertible Preferred Stock, shares		18,000,000
Ending balance	$ 53	$ 53	$ 48	$ 48	$ 53	$ 48
Ending balance, shares	532,000,000	531,000,000	478,000,000	477,000,000	532,000,000	478,000,000
OPIC
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 11,381	$ 11,530	$ 11,937	$ 10,001	$ 11,530	$ 10,001
Stock-based compensation	9	7	19	19
Stock Investment Plan and certain share-based benefit plans	21	1	19	5
Cash dividends declared on common stock		(202)		(343)
Cash dividends declared on preferred stock		(3)		(42)
Stock issuance				2,297
Conversion of Series A Convertible Preferred Stock		48
Ending balance	11,411	11,381	11,975	11,937	11,411	11,975
Amount of beneficial conversion			296
AOCI
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	36	41	86	142	41	142
Other comprehensive loss, net of tax	(5)	(5)	(13)	(56)
Ending balance	31	36	73	86	31	73
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(4,559)	(4,879)	(4,858)	(6,262)	(4,879)	(6,262)
Net income	312	320	299	1,369
Impact of adopting new accounting pronouncements				35
Ending balance	$ (4,247)	$ (4,559)	$ (4,559)	$ (4,858)	$ (4,247)	$ (4,559)
ASU 2016-01 | Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Effect of new accounting principle in period of adoption							$ 57
ASU 2018-02 | Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Effect of new accounting principle in period of adoption							$ 22