Components of deferred tax assets and liabilities (Details 3) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Deferred tax assets:
Postretirement	$ 41,298	$ 45,752
Compensation-related	35,196	37,917
Operating lease liabilities	25,718	26,710
Asset retirement obligations	9,687	8,696
Legal and environmental contingencies	8,526	8,603
Customer advances	7,615	7,683
Payroll tax deferral	0	6,940
Other	51,472	39,960
Total deferred tax assets	179,512	182,261
Deferred tax liabilities:
Basis differences on property, plant and equipment	608,528	585,095
Postretirement	47,340	48,302
Purchased gas adjustment	33,567	21,136
Operating lease right-of-use-assets	25,472	26,570
Intangible assets	23,007	21,074
Other	60,078	59,934
Total deferred tax liabilities	797,992	762,111
Valuation allowance	12,823	12,112
Net deferred income tax liability	$ 631,303	$ 591,962