Fair Value of Financial Instruments (Tables) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 133,104	$ 99,090
Restricted cash	0	0
Marketable securities	23,346	22,709
Total Financial Assets	156,450	121,799
Total debt	0	0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Restricted cash	11,043	10,926
Marketable securities	0	0
Total Financial Assets	11,043	10,926
Total debt	1,326,095	1,315,473
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	133,104	99,090
Restricted cash	11,043	10,926
Marketable securities	23,346	22,709
Total Financial Assets	167,493	132,725
Total debt	1,326,095	1,315,473
Interest Rate Swap [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liability for interest rate swap agreements	0	0
Interest Rate Swap [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liability for interest rate swap agreements	8,580	4,370
Interest Rate Swap [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liability for interest rate swap agreements	$ 8,580	$ 4,370