Components of deferred tax assets and liabilities (Details 3) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets:
Postretirement	$ 45,752	$ 51,495
Compensation-related	37,917	40,477
Operating lease liabilities	26,710	25,963
Asset retirement obligations	8,696	8,060
Legal and environmental contingencies	8,603	9,467
Customer advances	7,683	7,463
Payroll tax deferral	6,940	14,010
Other	39,960	37,944
Total deferred tax assets	182,261	194,879
Deferred tax liabilities:
Basis differences on property, plant and equipment	585,095	536,966
Postretirement	48,302	49,233
Operating lease right-of-use-assets	26,570	25,858
Intangible assets	21,074	19,514
Other	81,070	67,922
Total deferred tax liabilities	762,111	699,493
Valuation allowance	12,112	11,484
Net deferred income tax liability	$ 591,962	$ 516,098