Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Deferred tax assets:
Leases	$ 106,564		$ 0
Asset retirement obligations	9,957		7,797
Net operating loss carry-forwards	10,071	$ 10,100	12,612
Pension liabilities	3,161		2,873
Accruals and stock based compensation	1,935		6,545
Other	1,408		0
Total gross deferred tax assets	133,096		29,827
Less valuation allowance	0		(862)
Net deferred tax assets	133,096		28,965
Deferred tax liabilities:
Property, plant and equipment	110,297		99,902
Leases	105,475		0
Intangible assets	27,201		32,727
Prepaid assets and other	26,574		23,789
Total gross deferred tax liabilities	269,547		156,418
Net deferred tax liabilities	$ 136,451		$ 127,453