Consolidated Statement of Shareholder's Interest - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Stockholders' interest attributable to noncontrolling interest	$ 1		$ 0		$ 1	$ 0	$ 0
Total shareholder’s interest	11,714		11,367		11,714	11,367	10,604
Shareholder's Interest [Roll Forward]
Balance at beginning of period		$ 11,367		$ 10,604	11,367	10,604	9,653
Net income	352	306	411	312	1,363	1,705	1,479
Net Income (Loss) Attributable to Noncontrolling Interest					0	0	0
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest					1,363	1,705	1,479
Condolidation, Wholly Owned Subsidiary, Parent Ownership Interest, Purchase of Interest by Parent							3
Other comprehensive income/(loss), net of tax					(766)	(547)	(86)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent					(767)	(547)	(86)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest					1	0	0
Distributions					(250)	(395)	(445)
Balance at end of period	11,713		11,367		11,713	11,367	10,604
Shareholder's Interest [Member]
Shareholder's Interest [Roll Forward]
Balance at beginning of period		5,227		5,217	5,227	5,217	5,274
Net income					0	0	0
Condolidation, Wholly Owned Subsidiary, Parent Ownership Interest, Purchase of Interest by Parent							3
Other comprehensive income/(loss), net of tax					0	10	(60)
Distributions					0	0	0
Balance at end of period	5,227		5,227		5,227	5,227	5,217
Accumulated Other Comprehensive Income/(Loss) (Note11) [Member]
Shareholder's Interest [Roll Forward]
Balance at beginning of period		160		717	160	717	743
Net income					0	0	0
Condolidation, Wholly Owned Subsidiary, Parent Ownership Interest, Purchase of Interest by Parent							0
Other comprehensive income/(loss), net of tax					(767)	(557)	(26)
Distributions					0	0	0
Balance at end of period	(607)		160		(607)	160	717
Retained Earnings [Member]
Shareholder's Interest [Roll Forward]
Balance at beginning of period		$ 5,980		$ 4,670	5,980	4,670	3,636
Net income					1,363	1,705	1,479
Condolidation, Wholly Owned Subsidiary, Parent Ownership Interest, Purchase of Interest by Parent							0
Other comprehensive income/(loss), net of tax					0	0	0
Distributions					(250)	(395)	(445)
Balance at end of period	$ 7,093		$ 5,980		7,093	5,980	4,670
Noncontrolling Interest [Member]
Shareholder's Interest [Roll Forward]
Condolidation, Wholly Owned Subsidiary, Parent Ownership Interest, Purchase of Interest by Parent							0
Distributions					0	0	0
Parent [Member]
Shareholder's Interest [Roll Forward]
Condolidation, Wholly Owned Subsidiary, Parent Ownership Interest, Purchase of Interest by Parent							3
Distributions					$ (250)	$ (395)	$ (445)