Restructuring (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Asset impairments and restructuring
Restructuring charges	$ 2,446	$ 8,299	$ 2,446	$ 8,299
Other
Asset impairments and restructuring
Restructuring charges			300
U.S. Operations
Asset impairments and restructuring
Restructuring charges	1,300		800	700
Canadian and U.K. Operations
Asset impairments and restructuring
Restructuring charges	800		$ 1,300
Corporate Segment [Member]
Asset impairments and restructuring
Restructuring charges	$ 300			500
Wolverine, Brule and Willow Creek mines | Other
Asset impairments and restructuring
Restructuring charges		500
Wolverine, Brule and Willow Creek mines | U.S. Operations
Asset impairments and restructuring
Restructuring charges		700
Wolverine, Brule and Willow Creek mines | Canadian and U.K. Operations
Asset impairments and restructuring
Restructuring charges		$ 7,100		$ 7,100