Property, Plant and Equipment Assets Amortized by Units of Production (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Unproved Oil and Gas Property, Successful Effort Method	$ 1,116,282	$ 1,421,083
Property, Plant and Equipment, Gross	13,771,388	13,794,907
Assets amortized by units of production [Member]
Property, Plant and Equipment [Line Items]
Unproved Oil and Gas Property, Successful Effort Method	1,116,282	1,421,083
Coal Properties And Surface Lands	195,354	258,099
Mine development	7,771	7,930
Property Subject to or Available for Operating Lease, Gross	36,788	44,805
Prepaid Royalties	16,270	21,544
Advances for Gas Exploration, Development and Production Amount	13,762	19,745
Property, Plant and Equipment, Gross	1,405,920	1,797,880
Property, Plant and Equipment, Other, Gross	$ 19,693	$ 24,674