Tax reform (Details 3) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
TCJA revaluation related to accumulated other comprehensive loss balance		$ (39,500)	$ (42)	$ 7,735	$ 0
Deferred tax liabilities, regulatory assets			$ 13,776
Other comprehensive income (loss) [Member]
TCJA revaluation related to accumulated other comprehensive loss balance		7,700
Discontinued operations, held-for-sale [Member]
TCJA revaluation related to accumulated other comprehensive loss balance		1,000
Stranded costs [Member]
Deferred tax liabilities, regulatory assets	$ (4,200)	$ 285,500