Cash flow information (Details 1) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Supplemental Cash Flow Information [Abstract]
Interest, net of amounts capitalized and AFUDC - borrowed of $7.5 million and $8.6 million in 2015 and 2014, respectively	$ 70,203	$ 61,610
Income taxes paid, net	20,652	58,345
Property, plant and equipment additions in accounts payable	$ 15,499	$ 49,260