Fair value measurements (Details 3) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Carrying amount [Member]
Fair value, balance sheet grouping [Line Items]
Long-term debt	$ 2,376,802	$ 2,093,830	$ 2,185,917
Fair value [Member]
Fair value, balance sheet grouping [Line Items]
Long-term debt, fair value	$ 2,468,204	$ 2,238,548	$ 2,282,174