Debt and Capital Lease Obligations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Long term debt and capital lease obligations
Current	$ 5,485			$ 5,485		$ 55,983
Non-current	306,578			306,578		3,460
Interest expenses incurred for various debt instruments
Accretion of Franco Nevada royalty obligation	4,107	5,559
Accretion expense on royalty obligation discount	4,100		5,600	8,170	10,663
Amortization of debt issuance costs	539	251		1,064	508
Accretion of debt discount	0	629		576	1,241
Capitalized interest	32	(610)		(361)	(1,694)
Total interest expense, net of capitalized interest	10,930	7,557		20,662	14,227
3.25% Convertible Senior Notes due March 2028 [Member]
Long term debt and capital lease obligations
Current	0			0		48,081
Non-current	5,334			5,334		0
Interest expenses incurred for various debt instruments
Convertible debt, interest expense	43	395		380	791
7.875% Senior Notes due 2021 [Member]
Long term debt and capital lease obligations
Current	0			0		0
Non-current	300,000			300,000		0
Interest expenses incurred for various debt instruments
Interest Expense	5,906	0		10,041	0
Revolving Credit Facility [Member]
Interest expenses incurred for various debt instruments
Interest Expense	133	0		258	0
Kensington Term Facility [Member]
Interest expenses incurred for various debt instruments
Interest Expense	0	906		0	1,880
Capital Lease Obligations [Member]
Long term debt and capital lease obligations
Current	5,485			5,485		7,902
Non-current	1,244			1,244		3,460
Interest expenses incurred for various debt instruments
Interest Expense	98	265		266	608
Other debt obligations [Member]
Interest expenses incurred for various debt instruments
Interest Expense	$ 72	$ 162		$ 268	$ 230