Financing Receivables (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended			12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Middlemount Mine [Member]	Sep. 30, 2013 Middlemount Mine [Member]	Sep. 30, 2014 Middlemount Mine [Member]	Sep. 30, 2013 Middlemount Mine [Member]	Sep. 30, 2013 Codrilla Mine Project [Member]	Dec. 31, 2011 Codrilla Mine Project [Member]	Sep. 30, 2014 Codrilla Mine Project [Member]	Sep. 30, 2014 Financing Receivable [Member]	Dec. 31, 2013 Financing Receivable [Member]	Sep. 30, 2014 Financing Receivable [Member] Middlemount Mine [Member]	Dec. 31, 2013 Financing Receivable [Member] Middlemount Mine [Member]	Sep. 30, 2014 Financing Receivable [Member] Codrilla Mine Project [Member]	Dec. 31, 2013 Financing Receivable [Member] Codrilla Mine Project [Member]	Oct. 23, 2011 Coppabella, Moorvale, and Codrilla Mines [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net												$ 0	$ 0.1
Investments and other assets												358.3	351.7	329.2	323.2	29.1	28.5
Total financing receivables												358.3	351.8
Percentage of ownership before selldown										85.00%
Percentage of undivided interests acquired																		73.30%
Percentage of agreed sale price as final installment payment due											40.00%
Adjustment to discounted value of note receivable									1.6
Equity method investment, ownership percentage					50.00%		50.00%
Basis spread over Australian Bank Bill Swap Reference Rate							3.50%
Interest income	3.7	4.2	11.7	11.2	1.1	1.5	3.7	4.9
Additional interest income under a full accrual basis					$ 1.7	$ 1.5	$ 4.8	$ 4.8