Debt and Capital Lease Obligations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Debt Instrument [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage			0.50%
Long term debt and capital lease obligations
Debt, Current	$ 3,868		$ 3,868		$ 55,983
Long-term debt and capital leases	306,372		306,372		3,460
Interest expenses incurred for various debt instruments
Accretion of Franco Nevada royalty obligation	4,023	4,384	12,192	15,047
Amortization of debt issuance costs	540	1,331	1,604	1,838
Accretion of debt discount	0	639	576	1,879
Capitalized interest	(1,133)	(512)	(1,494)	(2,206)
Total interest expense, net of capitalized interest	9,662	7,351	30,324	21,578
3.25% Convertible Senior Notes due March 2028 [Member]
Long term debt and capital lease obligations
Debt, Current	0		0		48,081
Long-term debt and capital leases	5,334		5,334		0
Interest expenses incurred for various debt instruments
Convertible debt, interest expense	43	395	423	1,186
7.875% Senior Notes due 2021 [Member]
Long term debt and capital lease obligations
Debt, Current	0		0		0
Long-term debt and capital leases	300,000		300,000		0
Interest expenses incurred for various debt instruments
Interest Expense	5,906	0	15,947	0
Revolving Credit Facility [Member]
Interest expenses incurred for various debt instruments
Interest Expense	176	85	434	85
Kensington Term Facility [Member]
Interest expenses incurred for various debt instruments
Interest Expense	0	459	0	2,339
Capital Lease Obligations [Member]
Long term debt and capital lease obligations
Debt, Current	3,868		3,868		7,902
Long-term debt and capital leases	1,038		1,038		3,460
Interest expenses incurred for various debt instruments
Interest Expense	89	219	355	827
Other debt obligations [Member]
Interest expenses incurred for various debt instruments
Interest Expense	$ 18	$ 351	$ 287	$ 583