Fair Value Measurements Fair Value Measurements - Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended						48 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2014
Derivative [Line Items]
Net periodic cash settlements and accrued interest (1)	$ 177	[1]	$ 172	[1]	$ 170	[1]
Total expense for derivatives	17		34		140
Interest Expense	2,670		2,252		2,100		7,000
Reclassification out of Accumulated Other Comprehensive Income [Member]
Derivative [Line Items]
Interest Expense	$ 0		$ 4		$ 28

[1] The derivative settlements under the terms of the interest rate swap agreements are recognized as interest expense and are paid monthly.