Derivatives and Fair Value Measurements (Details 3) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Fair Value, Measurements, Recurring [Member]
Fair value of financial asset (liability) positions measured on a recurring basis
Investment in debt and equity securities	$ 41.4	$ 17.9
Commodity swaps and options - diesel fuel	95.3	40.3
Commodity swaps and options - explosives	(1.3)	(0.1)
Foreign currency hedge contracts	773.6	640.1
Total net financial assets	909.0	698.2
Fair Value, Measurements, Recurring [Member] | Level 1[Member]
Fair value of financial asset (liability) positions measured on a recurring basis
Investment in debt and equity securities	41.4	17.9
Commodity swaps and options - diesel fuel	0	0
Commodity swaps and options - explosives	0	0
Foreign currency hedge contracts	0	0
Total net financial assets	41.4	17.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair value of financial asset (liability) positions measured on a recurring basis
Investment in debt and equity securities	0	0
Commodity swaps and options - diesel fuel	95.3	40.3
Commodity swaps and options - explosives	(1.3)	(0.1)
Foreign currency hedge contracts	773.6	640.1
Total net financial assets	867.6	680.3
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair value of financial asset (liability) positions measured on a recurring basis
Investment in debt and equity securities	0	0
Commodity swaps and options - diesel fuel	0	0
Commodity swaps and options - explosives	0	0
Foreign currency hedge contracts	0	0
Total net financial assets	0	0
Carrying amount [Member]
Carrying amount of the Company's debt
Long-term debt	2,512.0	2,750.0
Estimated fair value [Member]
Carrying amount of the Company's debt
Long-term debt	$ 2,696.7	$ 2,960.0