Tax reform (Details 3) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
TCJA revaluation related to accumulated other comprehensive loss balance		$ 39,500	$ 0	$ (42)	$ 7,735
Deferred tax liabilities, regulatory assets			$ 11,904
Other comprehensive income (loss)
TCJA revaluation related to accumulated other comprehensive loss balance		(7,700)
Discontinued operations, held-for-sale
TCJA revaluation related to accumulated other comprehensive loss balance		$ (1,000)
Stranded costs
Deferred tax liabilities, regulatory assets	$ (4,200)