Income Tax Provision (Components of Income Tax Expense and Effective Income Tax Rate Reconciliation ) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Current
Federal									$ (15)	$ (33)	$ (26)
State									41	7	30
Foreign									6	9	10
Total Current									32	(17)	14
Deferred
Federal									721	830	981
State									69	123	151
Foreign									2	1	4
Total Deferred									792	954	1,136
Provision for income taxes	$ 189	$ 156	$ 200	$ 279	$ 109	$ 266	$ 279	$ 283	$ 824	$ 937	$ 1,150
Reconciliation Between U.S. Federal Statutory Tax Rate and Effective Tax Rate
Provision for income taxes at U.S. federal statutory tax rate									35.00%	35.00%	35.00%
State and local taxes (net of federal tax benefit)									3.20%	3.40%	3.40%
Other									0.00%	0.30%	(0.10%)
Effective tax rate									38.20%	38.70%	38.30%