Mineral Properties and Mine Development (Schedule of Mineral Properties) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Mining Properties, Mineral Rights and Mine Development [Member]
Property, Plant and Equipment [Line Items]
Mineral properties and mine development, gross	$ 1,010,155	$ 980,607
Accumulated depletion and amortization	(445,565)	(411,601)
Total mineral properties and mine development, net	564,590	569,006
Montana, United States of America [Member] | Mining Properties and Mineral Rights [Member] | Stillwater Mine [Member]
Property, Plant and Equipment [Line Items]
Mineral properties and mine development, gross	1,950	1,950
Montana, United States of America [Member] | Mine Development [Member] | Stillwater Mine [Member]
Property, Plant and Equipment [Line Items]
Mineral properties and mine development, gross	681,443	616,872
Montana, United States of America [Member] | Mine Development [Member] | East Boulder Mine [Member]
Property, Plant and Equipment [Line Items]
Mineral properties and mine development, gross	216,232	204,483
Ontario, Canada [Member] | Mining Properties and Mineral Rights [Member] | Marathon Property [Member]
Property, Plant and Equipment [Line Items]
Mineral properties and mine development, gross	8,560	55,332
San Juan, Argentina [Member] | Mining Properties and Mineral Rights [Member] | Altar Property [Member]
Property, Plant and Equipment [Line Items]
Mineral properties and mine development, gross	$ 101,970	$ 101,970