Fair Value Measurements - Financial Assets and Liabilities (Details) - Recurring - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Cash equivalents	$ 0	$ 44,091
Derivative assets, recorded in other current assets	9,187	2,235
Derivative liabilities, recorded in accrued expenses and deferred credits	(2,226)	(2,905)
Total	6,961	43,421
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Cash equivalents	0	0
Derivative assets, recorded in other current assets	0	0
Derivative liabilities, recorded in accrued expenses and deferred credits	0	0
Total	0	0
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Cash equivalents	0	44,091
Derivative assets, recorded in other current assets	9,187	2,235
Derivative liabilities, recorded in accrued expenses and deferred credits	(2,226)	(2,905)
Total	6,961	43,421
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Cash equivalents	0	0
Derivative assets, recorded in other current assets	0	0
Derivative liabilities, recorded in accrued expenses and deferred credits	0	0
Total	$ 0	$ 0