Uncompleted contracts - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Amounts included in contract liability at the beginning of the period	$ 28.2	$ 30.5
Contract with customer, performance obligation satisfied in previous period	8.0	$ 9.2
Remaining performance obligations	938.7
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-04-01 \| Revenue, Description Of Timing, 12 Months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations	$ 820.6
Remaining performance obligations, expected timing	12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-04-01 \| Revenue, Description Of Timing, 13 To 24 Months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations	$ 91.5
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-04-01 \| Revenue, Description Of Timing, 25 Months Or More
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations	$ 26.6
Remaining performance obligations, expected timing	25 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-04-01 \| Minimum \| Revenue, Description Of Timing, 13 To 24 Months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, expected timing	13 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-04-01 \| Maximum \| Revenue, Description Of Timing, 13 To 24 Months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, expected timing	24 months

Knife River Corp 10-Q