Consolidated Statement of Shareholder's Interest - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stockholders' interest attributable to noncontrolling interest	$ 0		$ 0		$ 0	$ 0	$ 1	$ 0
Total shareholder’s interest	15,884		12,803		15,884	12,803	11,714	$ 11,367
Shareholder's Interest [Roll Forward]
Balance at beginning of period		$ 12,803		$ 11,713	12,803	11,713	11,367
Net income	1,814	333	333	358	3,007	1,373	1,363
Net Income (Loss) Attributable to Noncontrolling Interest					0	0	0
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest					3,007	1,373	1,363
Other comprehensive income/(loss), net of tax					471	(283)	(766)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent					471	(283)	(767)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest					0	0	1
Distributions declared					(406)	(1)	(250)
Balance at end of period	15,884		12,803		15,884	12,803	11,713
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		(890)		(607)	(890)	(607)	160
Net income					0	0	0
Other comprehensive income/(loss), net of tax					471	(283)	(767)
Distributions declared					0	0	0
Balance at end of period	(419)		(890)		(419)	(890)	(607)
Retained Earnings [Member]
Shareholder's Interest [Roll Forward]
Balance at beginning of period		$ 8,466		$ 7,093	8,466	7,093	5,980
Net income					3,007	1,373	1,363
Other comprehensive income/(loss), net of tax					0	0	0
Distributions declared					(406)	0	(250)
Balance at end of period	11,076		$ 8,466		11,076	8,466	7,093
Noncontrolling Interest [Member]
Shareholder's Interest [Roll Forward]
Distributions declared					0	(1)	0
Parent [Member]
Shareholder's Interest [Roll Forward]
Distributions declared					(406)	$ 0	$ (250)
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