Fair-Value Measurements (Placement of Assets and Liabilities Within the Fair Value Hierarchy Levels) (Details) - Recurring Fair Value Measures - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Level 1 | Commodity Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value	$ 1,252	$ 559
Liabilities Fair Value	(4,463)	(2,066)
Level 1 | Preferred Distribution Rate Reset Election
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Fair Value	0	0
Level 2 | Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value	43	0
Liabilities Fair Value	(74)	0
Level 2 | Preferred Distribution Rate Reset Election
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Fair Value	0	0
Level 3 | Commodity Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value	0	0
Liabilities Fair Value	0	0
Level 3 | Preferred Distribution Rate Reset Election
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Fair Value	$ (49,409)	$ (45,209)