Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Operating activities:
Net income	$ 143,421	$ 137,230	$ 124,781
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation	23,384	21,051	19,220
Amortization of intangible assets	1,888	2,097	2,249
Deferred income taxes	1,626	(3,105)	1,060
Provision for credit losses	(56)	6,202	186
Amortization of debt issuance costs	263	0	0
Stock-based compensation costs	1,559	804	1,068
Unrealized gain on investments	(585)	0	0
Gain on sale of assets	(7,231)	(8,174)	(6,646)
Equity in earnings of unconsolidated affiliates, net of distributions	(8,055)	(4,946)	(402)
Changes in current assets and liabilities, net of acquisitions:
Receivables	(140,345)	28,944	(143,115)
Due from related-party	11,507	(489)	(296)
Contract assets, net	39,186	16,863	(91,686)
Inventories	(1,041)	(5,865)	(15,798)
Other current assets	(12,814)	(4,390)	(5,486)
Accounts payable	14,296	(21,782)	66,469
Due to related-party	(2,135)	(2,803)	(3,140)
Contract liabilities, net	67,196	15,713	8,043
Other current liabilities	20,728	(6,933)	15,992
Other noncurrent changes	10,585	921	2,006
Net cash provided by (used in) operating activities	163,377	171,338	(25,495)
Investing activities:
Capital expenditures	(48,278)	(35,590)	(35,844)
Net proceeds from sale or disposition of property	13,706	16,214	11,337
Investments	(2,489)	(596)	(58)
Net cash used in investing activities	(37,061)	(19,972)	(24,565)
Financing activities:
Proceeds under the credit facility	40,000	0	0
Repayments under the credit facility	(40,000)	0	0
Payment of debt issuance costs	(7,879)	0	0
Contribution from MDU Resources	13,531	0	0
Net amounts (paid to) received from related-party cash management program	(168,531)	(10,584)	131,077
Transfers to Centennial and MDU Resources	(178,992)	(69,327)	(106,573)
Net cash (used in) provided by financing activities	(41,871)	(151,911)	51,504
Increase (Decrease) in cash, cash equivalents and restricted cash	84,445	(545)	1,444
Cash, cash equivalents and restricted cash - beginning of year	1,567	2,112	668
Cash, cash equivalents and restricted cash - end of year	86,012	1,567	2,112
Supplemental Cash Flow Information:
Interest paid, net	14,800	16,845	5,748
Income taxes paid, net	50,207	52,322	33,201
Noncash investing activities:
Purchases of property, plant and equipment included in accounts payable	421	258	751
Related party
Financing activities:
Issuance of long-term debt	0	0	27,000
Repayment of related-party long-term notes payable	0	(45,000)	0
Repayment of related-party short-term notes payable	0	(27,000)	0
Nonrelated party
Financing activities:
Issuance of long-term debt	$ 300,000	$ 0	$ 0