Cash flow information (Details 1) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Supplemental Cash Flow Information [Abstract]
Interest, net of amounts capitalized and AFUDC - borrowed of $5.0 million and $5.7 million in 2015 and 2014, respectively	$ 45,102	$ 39,384
Income taxes paid, net	3,117	56,267
Property, plant and equipment additions in accounts payable	$ 13,467	$ 47,499