Fair Value Measurements (Details) $ in Thousands	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of domestic regions	2
Fair Value, Inputs, Level 2 [Member]
Assets:
Interest Rate Derivative Assets, at Fair Value	$ 9,501	$ 9,484
Total	10,358	12,750
Liabilities:
Interest rate derivative financial instruments	10,579	24,411
Total	10,579	24,411
Investments [Member] | Fair Value, Inputs, Level 2 [Member]
Assets:
Investments, Fair Value Disclosure	844	3,254
Demand Obligation [Member] | Fair Value, Inputs, Level 2 [Member]
Assets:
Investments, Fair Value Disclosure	$ 13	$ 12