Consolidated Statement of Shareholder's Interest - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stockholders' interest attributable to noncontrolling interest	$ 0		$ 0		$ 0	$ 0	$ 0	$ 1
Total shareholder’s interest	14,975		15,884		14,975	15,884	12,803	$ 11,714
Shareholder's Interest [Roll Forward]
Balance at beginning of period		$ 15,884		$ 12,803	15,884	12,803	11,713
Net income	526	701	1,814	333	2,224	3,007	1,373
Net Income (Loss) Attributable to Noncontrolling Interest					0	0	0
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest					2,224	3,007	1,373
Other comprehensive income/(loss), net of tax					(410)	471	(283)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent					(410)	471	(283)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest					0	0	0
Distributions declared					(2,723)	(406)	(1)
Balance at end of period	14,975		15,884		14,975	15,884	12,803
Shareholder's Interest [Member]
Shareholder's Interest [Roll Forward]
Balance at beginning of period		5,227		5,227	5,227	5,227	5,227
Net income					0	0	0
Other comprehensive income/(loss), net of tax					0	0	0
Distributions declared					0	0	0
Balance at end of period	5,227		5,227		5,227	5,227	5,227
Accumulated Other Comprehensive Income/(Loss) (Note11) [Member]
Shareholder's Interest [Roll Forward]
Balance at beginning of period		(419)		(890)	(419)	(890)	(607)
Net income					0	0	0
Other comprehensive income/(loss), net of tax					(410)	471	(283)
Distributions declared					0	0	0
Balance at end of period	(829)		(419)		(829)	(419)	(890)
Retained Earnings [Member]
Shareholder's Interest [Roll Forward]
Balance at beginning of period		$ 11,076		$ 8,466	11,076	8,466	7,093
Net income					2,224	3,007	1,373
Other comprehensive income/(loss), net of tax					0	0	0
Distributions declared					(2,723)	(406)	0
Balance at end of period	$ 10,577		11,076		10,577	11,076	8,466
Noncontrolling Interest [Member]
Shareholder's Interest [Roll Forward]
Distributions declared					0	0	(1)
Parent [Member]
Shareholder's Interest [Roll Forward]
Distributions declared					$ (2,723)	(406)	$ 0
Accounting Standards Update 2014-09 [Member]
Shareholder's Interest [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption			9			9
Accounting Standards Update 2014-09 [Member] | Shareholder's Interest [Member]
Shareholder's Interest [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption			0			0
Accounting Standards Update 2014-09 [Member] | Accumulated Other Comprehensive Income/(Loss) (Note11) [Member]
Shareholder's Interest [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption			0			0
Accounting Standards Update 2014-09 [Member] | Retained Earnings [Member]
Shareholder's Interest [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption			9			9
Accounting Standards Update 2014-09 [Member] | Noncontrolling Interest [Member]
Shareholder's Interest [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption			0			0
Accounting Standards Update 2014-09 [Member] | Parent [Member]
Shareholder's Interest [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption			$ 9			$ 9