Mineral Interests and Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Property, plant and equipment
Total property, plant and equipment	$ 2,618,440	$ 2,528,814
Less: Accumulated depreciation	(980,888)	(796,683)
Net property, plant and equipment	1,637,552	1,732,131
Mineral interests
Property, plant and equipment
Total property, plant and equipment	3,146,000	3,145,200
Less: Accumulated depreciation	(241,000)	(179,600)
Land
Property, plant and equipment
Total property, plant and equipment	79,733	87,088
Land improvements
Property, plant and equipment
Total property, plant and equipment	43,107	19,949
Buildings and leasehold improvements
Property, plant and equipment
Total property, plant and equipment	420,142	362,296
Mine development costs
Property, plant and equipment
Total property, plant and equipment	255,680	270,768
Machinery and equipment
Property, plant and equipment
Total property, plant and equipment	1,569,318	1,402,417
Gas properties and related development
Property, plant and equipment
Total property, plant and equipment	188,527	223,200
Construction in progress
Property, plant and equipment
Total property, plant and equipment	$ 61,933	$ 163,096