Asset Impairment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charge	$ 0	$ 46,772	$ 0	$ 46,772
Marathon Property [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment charge		46,800
Mineral properties, fair value		$ 8,600		$ 8,600