Fair Value Measurements (Details Textual) (USD $)	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair value of consolidated debt	$ 4,100,000,000	$ 4,400,000,000
Total indebtedness	3,888,284,000	4,135,139,000
Cash Flow Hedging [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative, Notional Amount	50,200,000	50,300,000
Average Remaining Maturity of Interest Rate Swaps	5 years 9 months
Period For Reclassification Into Earnings	12 months
Amount of unrealized gains (losses) estimated to be reclassified from accumulated other comprehensive loss to earnings during the next 12 months	(1,700,000)
Weighted average fixed rate of interest rate swaps	3.43%
Available-for-sale Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment Owned, Face Amount	100,900,000
Excepted Remaining Term of Available For Sale Securities	6 years 2 months
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized cost of the investment in available-for-sale debt securities	$ 64,600,000	$ 63,600,000