Supplemental Coal Data (Unaudited) (Details) MMcf in Thousands, $ in Thousands, T in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) T MMcf	Dec. 31, 2015 USD ($) T MMcf	Dec. 31, 2014 USD ($) T MMcf	Dec. 31, 2013 USD ($) T	Dec. 31, 2012 USD ($) T	Dec. 31, 2011 T
Reserve Quantities [Line Items]
Proved Oil and Gas Property, Successful Effort Method | $	$ 2,016,916	$ 1,922,602
Reserve Quantities [Roll Forward]
Proved Developed and Undeveloped Reserves, Production | MMcf	394,387	328,657	235,714
Net change due to revisions in quantity estimates | $	$ (35,502)	$ 408,006	$ 151,233
Unproved Oil and Gas Property, Successful Effort Method | $	$ 1,116,282	$ 1,421,083
Assigned Coal Reserves [Member]
Reserve Quantities [Line Items]
Coal Reserves	141
Steam [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Mass)	2,274
Coal Segment [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Mass)	2,361	3,047	3,238	3,032	4,229	4,314
Reserve Quantities [Roll Forward]
Purchase of minerals in place (Mass)	0	24	0	1	0
Sales of minerals in place (Mass)	(601)	(43)	(233)	(1,199)	(155)
Production of proved reserves (Mass)	(26)	(29)	(32)	(55)	(55)
Net change due to revisions in quantity estimates | $	$ (59,000)	$ (143,000)	$ 471,000	$ 56,000	$ 125,000
metallurgical coal [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Mass)	87
Affiliated Entity [Member]
Reserve Quantities [Line Items]
Proved Undeveloped Reserve (Mass)		0	0	55	57	41