Electric utility segment - Regulatory Proceedings (Details)			1 Months Ended		3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 25, 2019 contract	Apr. 27, 2017 USD ($)	May 31, 2019 USD ($)	Feb. 28, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 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
Call center, additional penalty accrued			$ 1,300,000
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