Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Assets:
Derivative Asset, Noncurrent	$ 15,987	$ 9,501
Fair Value, Inputs, Level 2 [Member]
Assets:
Interest Rate Derivative Assets, at Fair Value		9,501
Total	17,469	10,358
Liabilities:
Total	4,689	10,579
Investments [Member] | Fair Value, Inputs, Level 2 [Member]
Assets:
Investments, Fair Value Disclosure	1,469	844	$ 844
Demand Obligation [Member] | Fair Value, Inputs, Level 2 [Member]
Assets:
Investments, Fair Value Disclosure	$ 13	$ 13