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Changes in Operations Australia - Arrium (Details)
$ in Thousands, AUD in Millions
3 Months Ended 12 Months Ended 48 Months Ended
Mar. 31, 2017
USD ($)
Mar. 31, 2016
USD ($)
Dec. 31, 2017
AUD
Dec. 31, 2017
USD ($)
Dec. 31, 2014
USD ($)
Operating revenues $ 519,108 $ 482,616      
Australian Operations [Member]          
Asset Impairment Charges   13,000      
Operating revenues $ 73,907 51,798      
Arrium Limited [Member]          
Amount invested in capital project         $ 78,000
Arrium Limited [Member] | Scenario, Forecast [Member]          
Operating revenues     AUD 40 $ 31,000  
Arrium Limited [Member] | Australian Operations [Member]          
Provision for Doubtful Accounts   $ 8,100