Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets:
Postretirement	$ 28,953	$ 30,228
Environmental compliance	28,873	0
Compensation-related	27,363	19,867
Operating lease liabilities	14,242	13,914
Customer advances	8,312	7,615
Legal and environmental contingencies	4,881	8,265
Other	29,753	24,024
Total deferred tax assets	142,377	103,913
Deferred tax liabilities:
Basis differences on property, plant and equipment	421,212	405,428
Postretirement	39,110	47,340
Purchased gas adjustment	34,618	33,567
Environmental compliance	16,221	0
Operating lease right-of-use-assets	14,116	13,667
Intangible assets	12,756	12,032
Other	62,076	46,593
Total deferred tax liabilities	600,109	558,627
Valuation allowance	816	785
Net deferred income tax liability	$ 458,548	$ 455,499