Property, Plant and Equipment Assets Amortized by Units of Production (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Assets Amortized By Units of Production [Line Items]
Unproved Oil and Gas Property, Successful Effort Method	$ 1,463,406	$ 1,266,017
Coal Properties And Surface Lands	1,404,056	1,164,107
Mine development	354,607	262,511
Property Subject to or Available for Operating Lease, Gross	393,372	529,758
Prepaid Royalties	381,348	381,343
Airshafts	397,466	366,054
Advances for Gas Exploration, Development and Production Amount	22,668	8,229
Property, Plant and Equipment, Gross	13,578,509	12,121,557
Assets amortized by units of production [Member]
Assets Amortized By Units of Production [Line Items]
Unproved Oil and Gas Property, Successful Effort Method	1,487,166	1,266,017
Coal Properties And Surface Lands	273,242	317,676
Mine development	238,356	145,940
Property Subject to or Available for Operating Lease, Gross	99,506	118,697
Prepaid Royalties	48,043	55,749
Airshafts	38,794	21,866
Advances for Gas Exploration, Development and Production Amount	22,668	8,229
Property, Plant and Equipment, Gross	$ 2,207,775	$ 1,934,174