Cash flow information (Details) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Interest, net	$ 58,359,000	$ 58,119,000
Income taxes paid, net	8,887,000	46,430,000
AFUDC borrowed	1,800,000	676,000
Property, plant and equipment additions in accounts payable	34,594,000	16,914,000	$ 29,263,000
Issuance of common stock in connection with acquisition	17,993,000	0	$ 0
Continuing and discontinued operations
Income taxes paid, net	$ 5,100,000	$ 1,400,000