Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Deferred revenue	$ 0	$ 3,907
Net operating loss carry-forwards	12,612	14,983
Accruals and reserves	6,545	5,189
Pension benefits	2,873	3,556
Asset retirement obligations	7,797	4,608
Total gross deferred tax assets	29,827	32,243
Less valuation allowance	(862)	(862)
Net deferred tax assets	28,965	31,381
Deferred tax liabilities:
Deferred revenue	19,554	0
Plant-in-service	99,666	89,494
Intangible assets	32,963	37,682
Interest rate swaps	3,339	3,511
Other, net	896	1,573
Total gross deferred tax liabilities	156,418	132,260
Net deferred tax liabilities	$ 127,453	$ 100,879