Tax reform (Details 3) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
TCJA revaluation related to accumulated other comprehensive loss balance	$ (39,500,000)	$ 7,735,000	$ 0	$ 0
Deferred tax liabilities, regulatory assets		285,520,000
Effective income tax rate reconciliation, repatriation of foreign earnings, amount	447,000	$ 447,000
Other comprehensive income (loss) [Member]
TCJA revaluation related to accumulated other comprehensive loss balance	7,700,000
Discontinued operations, held-for-sale [Member]
TCJA revaluation related to accumulated other comprehensive loss balance	1,000,000.0
Stranded costs [Member]
Deferred tax liabilities, regulatory assets	$ (285,500,000)