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Goodwill and intangible assets (Narrative) (Details)
 12 Months Ended
Dec. 31, 2016
USD ($)
reporting_unit
Dec. 31, 2015
USD ($)
reporting_unit
Dec. 31, 2014
USD ($)
Finite-Lived Intangible Assets [Line Items]      
Number of reporting units | reporting_unit 6 6  
Impairment $ 0 $ 123,200,000 $ 0
Goodwill 652,743,000 669,036,000 798,481,000
Accumulated impairment loss on goodwill 168,800,000 168,800,000 45,600,000
Impairment of intangible assets 0   0
Amortization of intangible assets $ 26,124,000 27,295,000 $ 27,658,000
Amortization period 14 years    
Subsea Technologies      
Finite-Lived Intangible Assets [Line Items]      
Impairment   123,200,000  
Goodwill   73,000,000  
Trade names | Subsea Technologies      
Finite-Lived Intangible Assets [Line Items]      
Impairment of intangible assets   $ 1,900,000