Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Deferred revenues	$ 8,849	$ 2,367
Net operating loss carry-forwards	29,472	717
Accruals and reserves	10,517	5,658
Pension benefits	6,994	1,023
Asset retirement obligations	8,495	2,922
Intangible assets	0	325
Total gross deferred tax assets	64,327	13,012
Less valuation allowance	(709)	(709)
Net deferred tax assets	63,618	12,303
Deferred tax liabilities:
Plant-in-service	139,753	85,503
Intangibles	70,799	0
Interest rate swaps	4,433	271
Other, net	470	490
Total gross deferred tax liabilities	215,455	86,264
Net deferred tax liabilities	$ 151,837	$ 73,961