Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Leases	$ 123,129	$ 106,564
Asset retirement obligations	10,403	9,957
Net operating loss carry-forwards	7,723	10,071
Pension liabilities	3,868	3,161
Accruals and stock based compensation	3,093	1,935
Other	5,002	1,408
Total gross deferred tax assets	153,218	133,096
Less valuation allowance	0	0
Net deferred tax assets	153,218	133,096
Deferred tax liabilities:
Property, plant and equipment	127,602	110,676
Leases	126,458	105,475
Intangible assets	25,722	27,201
Prepaid assets and other	24,088	27,311
Total gross deferred tax liabilities	303,870	270,663
Net deferred tax liabilities	$ 150,652	$ 137,567