Derivative Instruments Accounts Receivable and Payable on a Gross and Net Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Offsetting Liabilities [Line Items]
Accounts Receivable Subject to Master Netting Arrangement	$ 2,391	$ 1,431
Accounts Payable Subject To Master Netting Arrangement	2,391	1,431
Net [Member]
Offsetting Liabilities [Line Items]
Accounts Receivable Subject to Master Netting Arrangement	5,892	1,333
Accounts Payable Subject To Master Netting Arrangement	14,252	3,904
Gross [Member]
Offsetting Liabilities [Line Items]
Accounts Receivable Subject to Master Netting Arrangement	8,283	2,764
Accounts Payable Subject To Master Netting Arrangement	$ 16,643	$ 5,335