Derivatives and Fair Value Measurements (Details 2) In Millions, unless otherwise specified	Dec. 31, 2014 Foreign currency forward contracts [Member] AUD	Dec. 31, 2013 Gross amounts of recognized assets/liabilities USD ($)	Dec. 31, 2013 Gross amounts of recognized assets/liabilities Commodity swap contracts [Member] USD ($)	Dec. 31, 2013 Gross amounts of recognized assets/liabilities Foreign currency forward contracts [Member] USD ($)	Dec. 31, 2013 Gross amounts offset in the consolidated balance sheet USD ($)	Dec. 31, 2013 Gross amounts offset in the consolidated balance sheet Commodity swap contracts [Member] USD ($)	Dec. 31, 2013 Gross amounts offset in the consolidated balance sheet Foreign currency forward contracts [Member] USD ($)	Dec. 31, 2014 Net amounts presented in the consolidated balance sheet USD ($)		Dec. 31, 2013 Net amounts presented in the consolidated balance sheet USD ($)	Dec. 31, 2014 Net amounts presented in the consolidated balance sheet Commodity swap contracts [Member] USD ($)		Dec. 31, 2013 Net amounts presented in the consolidated balance sheet Commodity swap contracts [Member] USD ($)	Dec. 31, 2014 Net amounts presented in the consolidated balance sheet Foreign currency forward contracts [Member] USD ($)		Dec. 31, 2013 Net amounts presented in the consolidated balance sheet Foreign currency forward contracts [Member] USD ($)	Dec. 31, 2013 Derivatives not offset in the consolidated balance sheet USD ($)		Dec. 31, 2013 Net amount of derivatives not offset USD ($)	Nov. 30, 2012 Previously monetized hedge contracts [Member] Foreign currency forward contracts [Member] AUD
Derivative [Line Items]
Derivative, Notional Amount	3,191.1																			1,900.0
Derivative asset, current		8.7	8.7	0	(2.3)	(2.3)	0			6.4			6.4			0	(5.0)	[1]	1.4
Derivative asset, noncurrent		3.0	2.8	0.2	(1.1)	(0.9)	(0.2)			1.9			1.9			0	(1.3)	[1]	0.6
Derivative liability, current		215.8	3.9	211.9	(1.9)	(1.9)	0	341.1	[2]	213.9	100.1	[2]	2.0	241.0	[2]	211.9	(5.4)	[1]	208.5
Derivative liability, noncurrent		$ 264.1	$ 2.5	$ 261.6	$ (1.5)	$ (1.3)	$ (0.2)	$ 236.0	[2]	$ 262.6	$ 67.0	[2]	$ 1.2	$ 169.0	[2]	$ 261.4	$ (0.9)	[1]	$ 261.7

[1] 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.

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