Fair Value (Details) (Fair value measurements, Recurring [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Cash and Cash Equivalents:
Cash and cash equivalents, fair value	$ 1,715	$ 1,634
Derivative Asset:
Total derivative instruments	116	95
Level 1 [Member]
Cash and Cash Equivalents:
Cash and cash equivalents, fair value	1,715	1,634
Derivative Asset:
Total derivative instruments
Level 2 [Member]
Cash and Cash Equivalents:
Cash and cash equivalents, fair value
Derivative Asset:
Total derivative instruments	116	95
Level 3 [Member]
Cash and Cash Equivalents:
Cash and cash equivalents, fair value
Derivative Asset:
Total derivative instruments
Interest rate swaps - fixed to floating rate [Member]
Derivative Asset:
Total derivative instruments	116	95
Interest rate swaps - fixed to floating rate [Member] | Level 1 [Member]
Derivative Asset:
Total derivative instruments
Interest rate swaps - fixed to floating rate [Member] | Level 2 [Member]
Derivative Asset:
Total derivative instruments	116	95
Interest rate swaps - fixed to floating rate [Member] | Level 3 [Member]
Derivative Asset:
Total derivative instruments
Prime money market funds [Member]
Cash and Cash Equivalents:
Cash and cash equivalents, fair value	1,715	1,634
Prime money market funds [Member] | Level 1 [Member]
Cash and Cash Equivalents:
Cash and cash equivalents, fair value	1,715	1,634
Prime money market funds [Member] | Level 2 [Member]
Cash and Cash Equivalents:
Cash and cash equivalents, fair value
Prime money market funds [Member] | Level 3 [Member]
Cash and Cash Equivalents:
Cash and cash equivalents, fair value