Changes in Operations Australia - Arrium (Details) $ in Thousands, AUD in Millions	3 Months Ended			9 Months Ended		12 Months Ended		48 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2017 AUD	Dec. 31, 2017 USD ($)	Dec. 31, 2014 USD ($)
Payments to Acquire Property, Plant, and Equipment				$ 149,105	$ 159,523
Operating revenues	$ 576,927	$ 501,002		1,636,468	1,484,993
Australian Operations [Member]
Asset Impairment Charges			$ 13,000
Operating revenues	$ 81,270	$ 54,150		$ 231,986	$ 161,231
Arrium Limited [Member] | Scenario, Forecast [Member]
Operating revenues						AUD 40	$ 32,000
Arrium Limited [Member] | Australian Operations [Member]
Payments to Acquire Property, Plant, and Equipment								$ 78,000
Provision for Doubtful Accounts			$ 8,100