Electric utility segment - Regulatory Proceedings (Details)		3 Months Ended	9 Months Ended
	Apr. 27, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($) $ / kWh
Electric utility subsidiary [Abstract]
Threshold of capital expenditures in excess of customer contributions for qualification for major project interim recovery	$ 2,500,000.0
MPIR requested amount	$ 3,400,000
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
Demand response, award percentage of annual maintenance costs			5.00%
Demand response, maximum award			$ 500,000
Demand response, penalty			$ 0
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		$ 3,500,000.0	$ 3,000,000
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