Cash flow information (Details) - USD ($)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest, net	[1]	$ 79,638,000	$ 87,920,000	$ 88,775,000
Income taxes paid, net	[2]	112,137,000	105,908,000	61,405,000
Capitalized interest and AFUDC borrowed		966,000	914,000	9,300,000
Property, plant and equipment additions in accounts payable		29,263,000	22,712,000	39,754,000
Continuing and discontinued operations [Member]
Income taxes paid, net		$ 9,700,000	$ 1,300,000	$ 2,400,000

[1] Capitalized interest and AFUDC - borrowed was $966,000, $914,000 and $9.3 million for the years ended December 31, 2017, 2016 and 2015, respectively.

[2] Income taxes paid, net of discontinued operations, were $9.7 million, $1.3 million and $2.4 million for the years ended December 31, 2017, 2016 and 2015, respectively.