Revenue from contracts with customers - Schedule of Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 313,627	$ 290,712	$ 1,340,324	$ 1,222,036
Other revenues	1,409	(1,018)	731	404
Total external operating revenues	315,036	289,694	1,341,055	1,222,440
Operating Segments
Disaggregation of Revenue [Line Items]
Total external operating revenues	319,535	293,660	1,389,191	1,265,850
Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(4,681)	(3,988)	(48,676)	(43,492)
Total external operating revenues	(4,681)	(3,988)	(48,676)	(43,492)
Natural gas transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	63,531	57,215	192,428	172,173
Natural gas storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	5,680	6,053	16,882	16,766
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	31,722	29,477	108,236	73,688
Residential utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	99,955	99,004	557,485	539,456
Commercial utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	98,073	84,335	448,008	393,874
Industrial utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	17,398	16,571	60,369	63,525
Other utility sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,949	2,045	5,592	6,046
Electric | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	118,676	111,641	335,284	321,300
Other revenues	(1,014)	(3,173)	(7,476)	(5,915)
Total external operating revenues	117,662	108,468	327,808	315,385
Electric | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(138)	(12)	(457)	(71)
Total external operating revenues	(138)	(12)	(457)	(71)
Electric | Natural gas transportation | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Electric | Natural gas storage | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Electric | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	19,118	16,182	53,832	44,998
Electric | Residential utility sales | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	35,730	37,076	103,984	107,735
Electric | Commercial utility sales | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	52,492	46,648	144,397	129,783
Electric | Industrial utility sales | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	9,525	9,702	27,936	32,809
Electric | Other utility sales | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,949	2,045	5,592	6,046
Natural gas distribution | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	141,862	131,468	882,169	788,319
Other revenues	2,353	2,122	8,088	6,200
Total external operating revenues	144,215	133,590	890,257	794,519
Natural gas distribution | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(92)	(31)	(278)	(108)
Total external operating revenues	(92)	(31)	(278)	(108)
Natural gas distribution | Natural gas transportation | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	16,292	15,279	49,816	43,194
Natural gas distribution | Natural gas storage | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Natural gas distribution | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,983	9,736	43,086	18,705
Natural gas distribution | Residential utility sales | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	64,225	61,928	453,501	431,721
Natural gas distribution | Commercial utility sales | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	45,581	37,687	303,611	264,091
Natural gas distribution | Industrial utility sales | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,873	6,869	32,433	30,716
Natural gas distribution | Other utility sales | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Pipeline | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	53,089	47,602	122,871	112,399
Other revenues	70	33	119	119
Total external operating revenues	53,159	47,635	122,990	112,518
Pipeline | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(4,269)	(3,924)	(47,401)	(43,249)
Total external operating revenues	(4,269)	(3,924)	(47,401)	(43,249)
Pipeline | Natural gas transportation | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	47,239	41,936	142,612	128,979
Pipeline | Natural gas storage | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	5,680	6,053	16,882	16,766
Pipeline | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,439	3,537	10,778	9,903
Pipeline | Residential utility sales | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Pipeline | Commercial utility sales | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Pipeline | Industrial utility sales | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Pipeline | Other utility sales | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	1	0	18
Other revenues	0	0	0	0
Total external operating revenues	0	1	0	18
Other | Intersegment eliminations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(182)	(21)	(540)	(64)
Total external operating revenues	(182)	(21)	(540)	(64)
Other | Natural gas transportation | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Natural gas storage | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Other | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	182	22	540	82
Other | Residential utility sales | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Commercial utility sales | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Industrial utility sales | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Other | Other utility sales | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 0	$ 0	$ 0	$ 0