Debt and Capital Lease Obligations (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Long term debt and capital lease obligations
Current	$ 6,130,000		$ 55,983,000
Non-current	307,791,000		3,460,000
Interest expenses incurred for various debt instruments
Accretion of Franco Nevada royalty obligation	4,062,000	5,104,000
Accretion expense on royalty obligation discount	4,100,000	5,100,000
Amortization of debt issuance costs	525,000	256,000
Accretion of debt discount	577,000	612,000
Capitalized interest	(393,000)	(1,083,000)
Total interest expense, net of capitalized interest	9,732,000	6,670,000
3.25% Convertible Senior Notes due March 2028 [Member]
Long term debt and capital lease obligations
Current	0		48,081,000
Non-current	5,334,000		0
Interest expenses incurred for various debt instruments
Convertible debt, interest expense	337,000	395,000
7.875% Senior Notes due 2021 [Member]
Long term debt and capital lease obligations
Current	0		0
Non-current	300,000,000		0
Interest expenses incurred for various debt instruments
Interest Expense	4,134,000	0
Revolving Credit Facility [Member]
Interest expenses incurred for various debt instruments
Interest Expense	125,000	0
Kensington Term Facility [Member]
Interest expenses incurred for various debt instruments
Interest Expense	0	974,000
Capital Lease Obligations [Member]
Long term debt and capital lease obligations
Current	6,130,000		7,902,000
Non-current	2,457,000		3,460,000
Interest expenses incurred for various debt instruments
Interest Expense	168,000	343,000
Other debt obligations [Member]
Interest expenses incurred for various debt instruments
Interest Expense	$ 197,000	$ 69,000