Capitalization - Changes in Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended						3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jul. 31, 2019	Sep. 30, 2018	Jul. 31, 2018	Mar. 31, 2018	Jan. 31, 2018	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ 7,269		$ 7,699		$ 3,925	$ 7,269	$ 6,932	$ 6,814	$ 7,699	$ 7,375	$ 3,925	$ 6,814	$ 3,925
Beginning balance, shares									511,915,450				511,915,450
Net income							391	312	$ 320	(458)	299	1,369	$ 1,023	1,210
Other comprehensive loss, net of tax							(4)	(5)	(5)	(12)	(13)	(56)	(14)	(81)
Stock-based compensation							7	9	7	10	19	19
Stock Investment Plan and certain share-based benefit plans							20	21	1	21	19	6
Cash dividends declared on common stock ($0.36/common share in January and March)							(411)		(202)	(368)		(343)
Cash dividends declared on preferred stock ($0.36/as-converted share in January and March)									(3)	(19)		(42)
Stock issuance												2,462
Impact of adopting new accounting pronouncements												35
Conversion of Series A Convertible Preferred Stock							0		0	0
Ending balance	$ 7,272		$ 6,873		$ 7,375		$ 7,272	7,269	$ 6,932	6,873	7,699	$ 7,375	$ 7,272	6,873
Ending balance, shares	540,311,707						540,311,707						540,311,707
Amount of beneficial conversion													$ 0	296
Amortization of beneficial conversion							$ 0			$ 35			0	296
Dividends declared (in usd per share)	$ 0.38	$ 0.38	$ 0.36	$ 0.36	$ 0.36	$ 0.36			$ 0.38
Preferred stock dividends declared (in usd per share)									$ 0.38	$ 0.36		$ 0.36
Series A Convertible Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ 21		$ 162		$ 0	$ 21	$ 21	$ 71	$ 162	$ 162	$ 0	$ 71	$ 0
Beginning balance, shares		200,000		1,600,000		0	200,000	200,000	700,000	1,600,000	1,600,000	0	700,000	0
Stock issuance												$ 162
Stock issuance, shares												1,600,000
Conversion of Series A Convertible Preferred Stock							$ (21)		$ (50)	$ (92)
Conversion of Series A Convertible Preferred Stock, shares							(200,000)		(500,000)	(900,000)
Ending balance	$ 0		$ 70		$ 162		$ 0	$ 21	$ 21	$ 70	$ 162	$ 162	$ 0	$ 70
Ending balance, shares	0		700,000		1,600,000		0	200,000	200,000	700,000	1,600,000	1,600,000	0	700,000
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ 53		$ 48		$ 44	$ 53	$ 53	$ 51	$ 48	$ 48	$ 44	$ 51	$ 44
Beginning balance, shares		532,000,000		478,000,000		445,000,000	532,000,000	531,000,000	512,000,000	478,000,000	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	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							$ 1		$ 2	$ 3
Conversion of Series A Convertible Preferred Stock, shares							(7,000,000)		(18,000,000)	(33,000,000)
Ending balance	$ 54		$ 51		$ 48		$ 54	$ 53	$ 53	$ 51	$ 48	$ 48	$ 54	$ 51
Ending balance, shares	540,000,000		512,000,000		477,000,000		540,000,000	532,000,000	531,000,000	512,000,000	478,000,000	477,000,000	540,000,000	512,000,000
OPIC
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ 11,411		$ 11,975		$ 10,001	$ 11,411	$ 11,381	$ 11,530	$ 11,975	$ 11,937	$ 10,001	$ 11,530	$ 10,001
Stock-based compensation							7	9	7	10	19	19
Stock Investment Plan and certain share-based benefit plans							20	21	1	21	19	5
Cash dividends declared on common stock ($0.36/common share in January and March)							(411)		(202)	(368)		(343)
Cash dividends declared on preferred stock ($0.36/as-converted share in January and March)									(3)	(19)		(42)
Stock issuance												2,297
Conversion of Series A Convertible Preferred Stock							20		48	89
Ending balance	$ 11,047		$ 11,708		$ 11,937		11,047	11,411	11,381	11,708	11,975	11,937	11,047	11,708
Amount of beneficial conversion										296
AOCI
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		31		73		142	31	36	41	73	86	142	41	142
Other comprehensive loss, net of tax							(4)	(5)	(5)	(12)	(13)	(56)
Ending balance	27		61		86		27	31	36	61	73	86	27	61
Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ (4,247)		$ (4,559)		$ (6,262)	(4,247)	(4,559)	(4,879)	(4,559)	(4,858)	(6,262)	(4,879)	(6,262)
Net income							391	312	320	(458)	299	1,369
Impact of adopting new accounting pronouncements												35
Ending balance	$ (3,856)		$ (5,017)		$ (4,858)		$ (3,856)	$ (4,247)	$ (4,559)	$ (5,017)	$ (4,559)	$ (4,858)	$ (3,856)	$ (5,017)
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