Cash flow information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Interest, net	$ 83,009,000	$ 79,638,000	$ 87,920,000
Income taxes paid, net	16,041,000	112,137,000	105,908,000
AFUDC borrowed	2,300,000	966,000	914,000
Property, plant and equipment additions in accounts payable	42,355,000	29,263,000	22,712,000
Issuance of common stock in connection with acquisition	18,186,000	0	0
Continuing and discontinued operations
Income taxes paid, net	$ 5,500,000	$ 9,700,000	$ 1,300,000