Derivatives and Fair Value Measurements (Details 1) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011		Dec. 31, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in other comprehensive income on derivative (effective portion)	$ 162.5	$ 223.6		$ 652.9
Gain (loss) reclassified from other comprehensive income into income (effective portion)	400.0	384.9		151.5
Gain (loss) reclassified from other comprehensive income into income (ineffective portion)	(4.7)	4.8		(1.1)
Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in income on non-designated derivatives	0	(32.8)	[1]	(8.5)	[2]
Interest rate swaps [Member] | Interest expense [Member] | Cash flow hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in other comprehensive income on derivative (effective portion)				0.8
Gain (loss) reclassified from other comprehensive income into income (effective portion)				(0.5)
Gain (loss) reclassified from other comprehensive income into income (ineffective portion)				0
Interest rate swaps [Member] | Interest expense [Member] | Cash flow hedges [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in income on non-designated derivatives				(8.5)	[2]
Commodity swaps and options [Member] | Operating costs and expenses [Member] | Cash flow hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in other comprehensive income on derivative (effective portion)	14.5	30.7		29.9
Gain (loss) reclassified from other comprehensive income into income (effective portion)	48.3	42.7		(36.2)
Gain (loss) reclassified from other comprehensive income into income (ineffective portion)	(4.7)	4.8		(1.1)
Commodity swaps and options [Member] | Operating costs and expenses [Member] | Cash flow hedges [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in income on non-designated derivatives	0	0		0
Diesel fuel hedge contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gains (losses) to be reclassified from comprehensive income to earnings within twelve months	14
U.S. explosives hedge contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gains (losses) to be reclassified from comprehensive income to earnings within twelve months	4
Foreign currency contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gains (losses) to be reclassified from comprehensive income to earnings within twelve months	260
Foreign currency contracts [Member] | Operating costs and expenses [Member] | Cash flow hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in other comprehensive income on derivative (effective portion)				622.2
Gain (loss) reclassified from other comprehensive income into income (effective portion)				188.2
Gain (loss) reclassified from other comprehensive income into income (ineffective portion)				0
Foreign currency contracts [Member] | Operating costs and expenses [Member] | Cash flow hedges [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in income on non-designated derivatives				0
Foreign currency contracts [Member] | Acquisition costs related to Macarthur Coal Limited [Member] | Economic hedge [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in other comprehensive income on derivative (effective portion)		0
Gain (loss) reclassified from other comprehensive income into income (effective portion)		0
Gain (loss) reclassified from other comprehensive income into income (ineffective portion)		0
Foreign currency contracts [Member] | Acquisition costs related to Macarthur Coal Limited [Member] | Economic hedge [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in income on non-designated derivatives		(32.8)	[1]
Operating Costs [Member] | Operating costs and expenses [Member] | Cash flow hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in other comprehensive income on derivative (effective portion)	148.0	193.4
Gain (loss) reclassified from other comprehensive income into income (effective portion)	351.7	342.2
Gain (loss) reclassified from other comprehensive income into income (ineffective portion)	0	0
Operating Costs [Member] | Operating costs and expenses [Member] | Cash flow hedges [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in income on non-designated derivatives	0	0
Capital Expenditures [Member] | Depreciation, depletion and amortization [Member] | Cash flow hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in other comprehensive income on derivative (effective portion)		(0.5)
Gain (loss) reclassified from other comprehensive income into income (effective portion)		0
Gain (loss) reclassified from other comprehensive income into income (ineffective portion)		0
Capital Expenditures [Member] | Depreciation, depletion and amortization [Member] | Cash flow hedges [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in income on non-designated derivatives		$ 0

[1] Relates to foreign currency contracts associated with the acquisition of PEA-PCI shares under the takeover process.

[2] Amounts relate to swaps that were de-designated and terminated in conjunction with the refinancing of the Company's previous credit facility.