Cash flow information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Interest, net*	$ 88,681,000	$ 93,414,000	$ 83,009,000
Income taxes paid (refunded), net**	65,536,000	(8,475,000)	16,041,000
AFUDC borrowed	2,600,000	2,800,000	2,300,000
Property, plant and equipment additions in accounts payable	26,082,000	46,119,000	42,355,000
Right-of-use assets obtained in exchange for new operating lease liabilities	54,356,000	54,880,000	0
Issuance of common stock in connection with acquisition	0	0	18,186,000
Debt assumed in connection with a business combination	0	1,163,000	0
Accrual for holdback payment related to a business combination	2,500,000	0	0
Continuing and discontinued operations
Income taxes paid (refunded), net**	$ 59,400,000	$ (9,400,000)	$ 5,500,000