Electric utility segment - Regulatory Proceedings (Details)			1 Months Ended	3 Months Ended	12 Months Ended
	Mar. 25, 2019 contract	Apr. 27, 2017 USD ($)	Feb. 28, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) $ / kWh
Electric utility subsidiary [Abstract]
MPIR requested amount		$ 3,600,000	$ 19,800,000
Decoupling order, service reliability performance, historical measurement period (in years)				10 years
Maximum penalty, percent of ROE					0.20%
Service reliability, pending adjusted maximum penalty amount				$ 6,700,000
Dead band percentage above or below the target				3.00%
Call center performance, maximum penalty percentage					0.08%
Call center performance, pending adjusted maximum penalty				$ 1,300,000
Call center performance, maximum penalty amount				2,100,000
Energy price, renewable projects with storage capacity (in dollars per kilowatt hour) | $ / kWh					0.115
Energy price, renewable energy-only projects (in dollars per kilowatt hour) | $ / kWh					0.095
Expected savings, phase 1, percent					20.00%
Performance incentive mechanism, incentive cap				3,000,000	$ 3,500,000
Expected savings, Phase 2, January 2019					15.00%
Expected savings, Phase 2, February 2019					10.00%
Expected savings, Phase 2, March 2019					5.00%
Performance incentive mechanism penalty					$ 0
Number of contracts which qualified for incentives under Phase 1 | contract	6
Incentives accrued				$ 1,700,000