Consolidated financial statements (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Impact of revision [Line Items]
Other current assets	$ 39,251		$ 42,605
Total current assets	332,993		323,985
Other non-current assets	118,273		113,812
Short-term borrowings			0
Total current liabilities	294,971		317,043
Long-term debt	612,787		627,973
Proceeds from short-term borrowings	0	$ 75,000
Proceeds from issuing long-term debt	107,000	211,500
Payments on long-term debt, including costs of debt reacquisition	$ (128,584)	(346,021)
As Previously Reported [Member]
Impact of revision [Line Items]
Other current assets			44,608
Total current assets			325,988
Other non-current assets			111,809
Short-term borrowings			434,000
Total current liabilities			751,043
Long-term debt			193,973
Proceeds from short-term borrowings		148,000
Proceeds from issuing long-term debt		0
Payments on long-term debt, including costs of debt reacquisition		(207,521)
Adjustment [Member]
Impact of revision [Line Items]
Other current assets			(2,003)
Total current assets			(2,003)
Other non-current assets			2,003
Short-term borrowings			(434,000)
Total current liabilities			(434,000)
Long-term debt			$ 434,000
Proceeds from short-term borrowings		(73,000)
Proceeds from issuing long-term debt		211,500
Payments on long-term debt, including costs of debt reacquisition		$ (138,500)