Consolidated Statement of Shareholder's Interest (USD $) In Millions, unless otherwise specified	3 Months Ended				12 Months Ended
	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Shareholder's Interest [Roll Forward]
Balance at beginning of period		$ 8,896		$ 10,322	$ 8,896	$ 10,322	$ 10,980
Net income	268	295	611	451	1,214	1,795	1,948
Other comprehensive income/(loss), net of tax					143	(221)	(106)
Distributions					(600)	(3,000)	(2,500)
Balance at end of period	9,653		8,896		9,653	8,896	10,322
Shareholder's Interest [Member]
Shareholder's Interest [Roll Forward]
Balance at beginning of period		5,274		5,274	5,274	5,274	5,149
Net income					0	0	0
Other comprehensive income/(loss), net of tax					0	0	125
Distributions					0	0	0
Balance at end of period	5,274		5,274		5,274	5,274	5,274
Accumulated Other Comprehensive Income/(Loss) (Note12) [Member]
Shareholder's Interest [Roll Forward]
Balance at beginning of period		600		821	600	821	1,052
Net income					0	0	0
Other comprehensive income/(loss), net of tax					143	(221)	(231)
Distributions					0	0	0
Balance at end of period	743		600		743	600	821
Retained Earnings [Member]
Shareholder's Interest [Roll Forward]
Balance at beginning of period		3,022		4,227	3,022	4,227	4,779
Net income					1,214	1,795	1,948
Other comprehensive income/(loss), net of tax					0	0	0
Distributions					(600)	(3,000)	(2,500)
Balance at end of period	$ 3,636		$ 3,022		$ 3,636	$ 3,022	$ 4,227