Fair Value Measurements Fair Value of Derivatives by Balance Sheet Location (Details) (USD $)	3 Months Ended	12 Months Ended
	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Mar. 31, 2011
Derivatives Fair Value [Line Items]
Maximum maturity of outstanding cash flow hedges, in months		36 months
Gain (loss) recognized in earnings for previously terminated interest rate swaps		$ 0	$ 0
Gain related to ineffective portion of hedging relationships		5,000,000	0
Unamortized losses on senior notes		4,000,000	5,000,000
Notional Amount of Interest Rate Fair Value Hedge Derivatives				850,000,000
Proceeds related to termination of interest rate swap contracts	80,000,000
Increase (Decrease) in Accrued Interest Receivable, Net	5,000,000
Principal amount of debt hedged by interest rate derivative contract		850,000,000
Deferred Gain (Loss) on Discontinuation of Interest Rate Fair Value Hedge		72,000,000
Notional Amount of Interest Rate Derivatives		0	0
Unamortized gains on senior notes		1,000,000	2,000,000
Unrealized gain on interest rate cash flow hedges, pretax, AOCI		7,000,000	8,000,000
Derivative Assets		87,000,000	82,000,000
Derivative Liabilities		131,000,000	189,000,000
Gain (loss) on previously terminated interest rate swaps to be reclassified within twelve months		9,000,000
Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative Instruments in Hedges, Assets, at Fair Value		51,000,000	59,000,000
Derivative Instruments in Hedges, Liabilities, at Fair Value		118,000,000	158,000,000
Designated as Hedging Instrument [Member] | Prepaid And Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Instruments in Hedges, Assets, at Fair Value		31,000,000	32,000,000
Designated as Hedging Instrument [Member] | Other Long Term Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Instruments in Hedges, Assets, at Fair Value		20,000,000	27,000,000
Designated as Hedging Instrument [Member] | Other current liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Instruments in Hedges, Liabilities, at Fair Value		69,000,000	87,000,000
Designated as Hedging Instrument [Member] | Other Long Term Liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Instruments in Hedges, Liabilities, at Fair Value		49,000,000	71,000,000
Not Designated as Hedging Instrument [Member] | Prepaid And Other Current Assets [Member]
Derivatives Fair Value [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Asset, at Fair Value		36,000,000	23,000,000
Not Designated as Hedging Instrument [Member] | Other current liabilities [Member]
Derivatives Fair Value [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value		$ 13,000,000	$ 31,000,000