Cash flow information (Details 13) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accounting Policies [Abstract]
Interest, net of amount capitalized and AFUDC - borrowed of $9,809, $11,608 and $8,800 in 2015, 2014 and 2013, respectively	$ 90,386	$ 81,241	$ 81,575
Income taxes paid, net	33,409	64,211	52,580
Capital expenditures incurred but not yet paid	51,702	62,453	22,832
Capitalized interest and AFUDC borrowed	$ 9,809	$ 11,608	$ 8,800