Fair Value Measurements (Details Textual) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair value of consolidated debt	$ 4,300,000	$ 4,000,000
Total indebtedness	4,055,541	3,849,141
Cash Flow Hedging [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notional amount of interest rate swaps	$ 49,600	49,900
Average Remaining Maturity of Interest Rate Swaps	4 years 3 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	$ 1,300
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	4 years 8 months
Amortized cost of the investment in available-for-sale debt securities	$ 71,300	67,800
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage-backed securities Available-for-sale, fair value disclosure	$ 74,600	$ 65,500