Fair Value of Financial Instruments (Tables) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	$ 113,128	$ 104,677
Restricted Cash Fair Value Disclosure	0	0
Available-for-sale Securities	23,800	22,264
Assets, Fair Value Disclosure	136,928	126,941
Long-term Debt, Fair Value	0	0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Restricted Cash Fair Value Disclosure	11,706	12,360
Available-for-sale Securities	0	0
Assets, Fair Value Disclosure	11,706	12,360
Long-term Debt, Fair Value	1,303,256	1,323,201
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	113,128	104,677
Restricted Cash Fair Value Disclosure	11,706	12,360
Available-for-sale Securities	23,800	22,264
Assets, Fair Value Disclosure	148,634	139,301
Long-term Debt, Fair Value	1,303,256	1,323,201
Interest Rate Swap [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative Liability	0	0
Interest Rate Swap [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative Liability	5,510	3,781
Interest Rate Swap [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative Liability	$ 5,510	$ 3,781