Derivative Instruments Accounts Receivable and Payable on a Gross and Net Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Offsetting Liabilities [Line Items]
Accounts Receivable Subject to Master Netting Arrangement	$ 3,834	$ 2,391
Accounts Payable Subject To Master Netting Arrangement	3,834	2,391
Net [Member]
Offsetting Liabilities [Line Items]
Accounts Receivable Subject to Master Netting Arrangement	8,534	5,892
Accounts Payable Subject To Master Netting Arrangement	20,907	14,252
Gross [Member]
Offsetting Liabilities [Line Items]
Accounts Receivable Subject to Master Netting Arrangement	12,368	8,283
Accounts Payable Subject To Master Netting Arrangement	$ 24,741	$ 16,643