Cash flow information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Interest, net	$ 93,414,000	$ 83,009,000	$ 79,638,000
Income taxes paid (refunded), net	(8,475,000)	16,041,000	112,137,000
AFUDC borrowed	2,800,000	2,300,000	966,000
Property, plant and equipment additions in accounts payable	46,119,000	42,355,000	29,263,000
Issuance of common stock in connection with acquisition	0	18,186,000	0
Debt assumed in connection with a business combination	1,163,000
Right-of-use assets obtained in exchange for new operating lease liabilities	54,880,000
Continuing and discontinued operations
Income taxes paid (refunded), net	$ (9,400,000)	$ 5,500,000	$ 9,700,000