Consolidated Statement of Shareholder's Interest - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Shareholder's Interest [Roll Forward]
Balance at beginning of period	$ 15,884	$ 12,803
Net income	701	333
Other Comprehensive Income (Loss), Net of Tax	113	90
Cumulative Effect of New Accounting Principle in Period of Adoption		9
Distributions declared	(1,013)	(28)
Balance at end of period	15,685	13,207
Shareholder's Interest [Member]
Shareholder's Interest [Roll Forward]
Balance at beginning of period	5,227	5,227
Net income	0	0
Other Comprehensive Income (Loss), Net of Tax	0	0
Cumulative Effect of New Accounting Principle in Period of Adoption		0
Distributions declared	0	0
Balance at end of period	5,227	5,227
Accumulated Other Comprehensive Income/(Loss) (Note11) [Member]
Shareholder's Interest [Roll Forward]
Balance at beginning of period	(419)	(890)
Net income	0	0
Other Comprehensive Income (Loss), Net of Tax	113	90
Distributions declared	0	0
Balance at end of period	(306)	(800)
Accumulated Other Comprehensive Income/(Loss) (Note11) [Member] | Accounting Standards Update 2014-09 [Member]
Shareholder's Interest [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption		0
Retained Earnings [Member]
Shareholder's Interest [Roll Forward]
Balance at beginning of period	11,076	8,466
Net income	701	333
Other Comprehensive Income (Loss), Net of Tax	0	0
Distributions declared	(1,013)	(28)
Balance at end of period	$ 10,764	8,780
Retained Earnings [Member] | Accounting Standards Update 2014-09 [Member]
Shareholder's Interest [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption		$ 9