Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Deferred revenues	$ 3,907	$ 8,849
Net operating loss carry-forwards	14,983	29,472
Accruals and reserves	5,189	10,517
Pension benefits	3,556	6,994
Asset retirement obligations	4,608	8,495
Total gross deferred tax assets	32,243	64,327
Less valuation allowance	(862)	(709)
Net deferred tax assets	31,381	63,618
Deferred tax liabilities:
Plant-in-service	89,494	139,753
Intangible assets	37,682	70,799
Interest rate swaps	3,511	4,433
Other, net	1,573	470
Total gross deferred tax liabilities	132,260	215,455
Net deferred tax liabilities	$ 100,879	$ 151,837