Cash flow information (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Interest, net	$ 19,067,000	$ 17,910,000
Income taxes paid (refunded), net	29,000	(1,056,000)
AFUDC borrowed	556,000	382,000
Right-of-use assets obtained in exchange for new operating lease liabilities	2,731,000	0	$ 0
Property, plant and equipment additions in accounts payable	27,655,000	16,829,000	42,355,000
Debt assumed in connection with a business combination	1,163,000	0	0
Issuance of common stock in connection with a business combination	0	0	$ 18,186,000
Continuing and discontinued operations
Income taxes paid (refunded), net	$ 132,000	$ 1,700,000