Derivatives and Fair Value Measurements (Details 2) AUD in Millions, $ in Millions		Dec. 31, 2014 USD ($)	[1]	Dec. 31, 2014 AUD	Dec. 31, 2013 USD ($)	Nov. 30, 2012 AUD
Foreign currency forward contracts [Member]
Derivative [Line Items]
Derivative, Notional Amount | AUD				AUD 3,191.1
Gross amounts of recognized assets/liabilities
Derivative [Line Items]
Derivative asset, current					$ 8.7
Derivative asset, noncurrent					3.0
Derivative liability, current					215.8
Derivative liability, noncurrent					264.1
Gross amounts of recognized assets/liabilities | Commodity swap contracts [Member]
Derivative [Line Items]
Derivative asset, current					8.7
Derivative asset, noncurrent					2.8
Derivative liability, current					3.9
Derivative liability, noncurrent					2.5
Gross amounts of recognized assets/liabilities | Foreign currency forward contracts [Member]
Derivative [Line Items]
Derivative asset, current					0.0
Derivative asset, noncurrent					0.2
Derivative liability, current					211.9
Derivative liability, noncurrent					261.6
Gross amounts offset in the consolidated balance sheet
Derivative [Line Items]
Derivative asset, current					(2.3)
Derivative asset, noncurrent					(1.1)
Derivative liability, current					(1.9)
Derivative liability, noncurrent					(1.5)
Gross amounts offset in the consolidated balance sheet | Commodity swap contracts [Member]
Derivative [Line Items]
Derivative asset, current					(2.3)
Derivative asset, noncurrent					(0.9)
Derivative liability, current					(1.9)
Derivative liability, noncurrent					(1.3)
Gross amounts offset in the consolidated balance sheet | Foreign currency forward contracts [Member]
Derivative [Line Items]
Derivative asset, current					0.0
Derivative asset, noncurrent					(0.2)
Derivative liability, current					0.0
Derivative liability, noncurrent					(0.2)
Net amounts presented in the consolidated balance sheet
Derivative [Line Items]
Derivative asset, current					6.4
Derivative asset, noncurrent					1.9
Derivative liability, current		$ 341.1			213.9
Derivative liability, noncurrent		236.0			262.6
Net amounts presented in the consolidated balance sheet | Commodity swap contracts [Member]
Derivative [Line Items]
Derivative asset, current					6.4
Derivative asset, noncurrent					1.9
Derivative liability, current		100.1			2.0
Derivative liability, noncurrent		67.0			1.2
Net amounts presented in the consolidated balance sheet | Foreign currency forward contracts [Member]
Derivative [Line Items]
Derivative asset, current					0.0
Derivative asset, noncurrent					0.0
Derivative liability, current		241.0			211.9
Derivative liability, noncurrent		$ 169.0			261.4
Derivatives not offset in the consolidated balance sheet
Derivative [Line Items]
Derivative asset, current	[2]				(5.0)
Derivative asset, noncurrent	[2]				(1.3)
Derivative liability, current	[3]				(5.4)
Derivative liability, noncurrent	[3]				(0.9)
Net amount of derivatives not offset
Derivative [Line Items]
Derivative asset, current					1.4
Derivative asset, noncurrent					0.6
Derivative liability, current					208.5
Derivative liability, noncurrent					$ 261.7
Previously monetized hedge contracts [Member] | Foreign currency forward contracts [Member]
Derivative [Line Items]
Derivative, Notional Amount | AUD						AUD 1,900.0

[1] All commodity swap contracts and foreign currency forward contracts were in a liability position as of December 31, 2014.

[2] Adjustments relate to the further netting of derivative contracts with a common counterparty across the Company's foreign currency, diesel fuel and explosives hedging strategy derivative contract portfolios that would be contractually enforceable in the event of default.

[3] Adjustments relate to the further netting of derivative contracts with a common counterparty across the Company's foreign currency, diesel fuel and explosives hedging strategy derivative contract portfolios that would be contractually enforceable in the event of default.