Fair Value Measurements (Details Textual) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fair value of debt		$ 4,590,000,000	$ 4,280,000,000
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss to reflect changes in fair value	$ 18,000,000