Fair value measurements (Details 4) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Carrying amount [Member]
Fair value, balance sheet grouping [Line Items]
Long-term debt	$ 1,901,948	$ 1,796,163	$ 2,200,773
Fair value [Member]
Fair value, balance sheet grouping [Line Items]
Long-term debt, fair value	$ 2,047,339	$ 1,819,828	$ 2,277,074