Fair Value Measurements (Details Textual) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair value of consolidated debt	$ 4,000,000	$ 4,400,000
Total indebtedness	3,866,007	4,135,139
Cash Flow Hedging [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notional amount of interest rate swaps	$ 50,100	50,300
Average Remaining Maturity of Interest Rate Swaps	5 years 6 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)
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]
Face amount of investment in available-for-sale debt securities	$ 100,900
Excepted Remaining Term of Available For Sale Securities	5 years 11 months
Amortized cost of the investment in available-for-sale debt securities		$ 63,600
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	$ 65,700