Supplementary Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Change in working capital accounts:
Accounts receivable	$ (5.9)	$ 20.2
Product inventory	0.6	(17.4)
Material and supplies inventory	(2.6)	1.4
Prepaid expense and other current assets	(7.8)	(2.2)
Income and mining taxes receivable	4.6	0.7
Accounts payable and accrued liabilities	(7.1)	(17.8)
Income and mining taxes payable	0	1.2
Change in working capital accounts, total	(18.2)	(13.9)
Cash interest paid	7.9	0.2
Income and mining taxes paid, net of refunds	(1.1)	2.2
Refund of certain state income taxes related to prior year tax returns	7.9
Investing activities
Change in capital expenditure accrual	0	20.3
Other investing adjustments	$ 23.7	$ 6.6