Emergence from Bankruptcy and Fresh Start Accounting (Accounting Impact of Emergence) (Details) $ in Thousands	Oct. 01, 2016 USD ($)
Reorganizations [Abstract]
Percentage of voting shares received upon emergence of new entity (percent)	50.00%
Assets
Fresh-Start Adjustment, Increase (Decrease), Property and Equipment, Net	$ (2,363,829)
Fresh Start Adjustments
Assets
Fresh-Start Adjustment, Increase (Decrease), Receivables, Net	779
Fresh-Start Adjustment, Increase (Decrease), Inventories	(21,078)
Fresh-Start Adjustment, Increase (Decrease), Prepaid and Other Current Assets	53,851
Fresh-Start Adjustment, Increase (Decrease), Current Assets	33,552
Fresh-Start Adjustment, Increase (Decrease), Property and Equipment, Net	(2,363,829)
Fresh-Start Adjustment, Increase (Decrease), Prepaid Royalties	(20,997)
Fresh-Start Adjustment, Increase (Decrease), Equity Investments	(61,606)
Fresh-Start Adjustment, Increase (Decrease), Other Assets, Noncurrent	37,503
Fresh-Start Adjustment, Increase (Decrease), Other Assets	(45,100)
Fresh-Start Adjustment, Increase (Decrease), Assets	(2,375,377)
Liabilities not subject to compromise
Fresh-Start Adjustment, Increase (Decrease), Accounts Payable	(250)
Fresh-Start Adjustment, Increase (Decrease), Accrued Liabilities and Other Current Liabilities	26,644
Fresh-Start Adjustment, Increase (Decrease), Current Liabilities	26,394
Fresh-Start Adjustment, Increase (Decrease), Asset Retirement Obligations	(60,570)
Fresh-Start Adjustment, Increase (Decrease), Pension Obligations	24,565
Fresh-Start Adjustment, Increase (Decrease), Other Postretirement Obligations	24,836
Fresh-Start Adjustment, Increase (Decrease), Accrued Workers' Compensation	74,520
Fresh-Start Adjustment, Increase (Decrease), Noncurrent Other Obligations	888
Fresh-Start Adjustment, Increase (Decrease), Other Noncurrent Obligations	90,633
Fresh-Start Adjustment, Increase (Decrease), Liabilities	90,633
Stockholders’ equity (deficit)
Fresh-Start Adjustment, Increase (Decrease), Retained Earnings (Deficit)	(2,467,716)
Fresh-Start Adjustment, Increase (Decrease), Accumulated Other Comprehensive Income (Loss)	1,706
Fresh-Start Adjustment, Increase (Decrease), Stockholders' Equity	(2,466,010)
Fresh-Start Adjustment, Increase (Decrease), Liabilities and Stockholders' Equity	(2,375,377)
Effect of Plan
Assets
Fresh-Start Adjustment, Increase (Decrease), Cash and Cash Equivalents	(199,718)
Fresh-Start Adjustment, Increase (Decrease), Prepaid and Other Current Assets	6,367
Fresh-Start Adjustment, Increase (Decrease), Current Assets	(193,351)
Fresh-Start Adjustment, Increase (Decrease), Other Assets, Noncurrent	34,495
Fresh-Start Adjustment, Increase (Decrease), Other Assets	34,495
Fresh-Start Adjustment, Increase (Decrease), Assets	(158,856)
Liabilities not subject to compromise
Fresh-Start Adjustment, Increase (Decrease), Accrued Liabilities and Other Current Liabilities	(36,331)
Fresh-Start Adjustment, Increase (Decrease), Current Maturities of Long-term Debt	3,265
Fresh-Start Adjustment, Increase (Decrease), Current Liabilities	(33,066)
Fresh-Start Adjustment, Increase (Decrease), Long-term Debt	323,235
Fresh-Start Adjustment, Increase (Decrease), Other Noncurrent Obligations	290,169
Fresh-Start Adjustment, Increase (Decrease), Liabilities Subject to Compromise	(5,278,612)
Fresh-Start Adjustment, Increase (Decrease), Liabilities	(4,988,443)
Stockholders’ equity (deficit)
Fresh-Start Adjustment, Increase (Decrease), Preferred and Common Stock Held in Treasury	53,863
Fresh-Start Adjustment, Increase (Decrease), Retained Earnings (Deficit)	7,146,693
Fresh-Start Adjustment, Increase (Decrease), Stockholders' Equity	4,829,587
Fresh-Start Adjustment, Increase (Decrease), Liabilities and Stockholders' Equity	(158,856)
Predecessor
Preconfirmation
Preconfirmation, Cash and cash equivalents	400,205
Preconfirmation, Short term investments	111,451
Preconfirmation, Restricted cash	81,563
Preconfirmation, Trade accounts receivable	165,522
Preconfirmation, Other Receivables	17,227
Preconfirmation, Inventories	159,410
Preconfirmation, Prepaid Royalties, Current	4,805
Preconfirmation, Deferred Income Tax Assets, Current	0
Preconfirmation, Coal Derivative Assets	2,180
Preconfirmation, Prepaid and Other Current Assets	36,960
Preconfirmation, Current Assets	979,323
Preconfirmation, Property and Equipment, Net	3,434,941
Preconfirmation, Prepaid Royalties, Noncurrent	20,997
Preconfirmation, Investments	164,232
Preconfirmation, Other Assets, Noncurrent	58,569
Preconfirmation, Other Assets	243,798
Preconfirmation, Assets	4,658,062
Preconfirmation, Accounts Payable	74,595
Precomfirmation, Accrued expenses and other current liabilities	225,739
Preconfirmation, Current Maturities of Long-term Debt	3,397
Preconfirmation, Current Liabilities	303,731
Preconfirmation, Long-term Debt	30,037
Preconfirmation, Asset Retirement Obligations	394,699
Preconfirmation, Pension Obligations	23,716
Preconfirmation, Other Postretirement Obligations	87,123
Preconfirmation, Accrued Workers' Compensation	119,828
Preconfirmation, Deferred Income Tax Liabilities	0
Preconfirmation, Noncurrent Other Obligations	96,410
Preconfirmation, Liabilities Not Subject to Compromise	1,055,544
Preconfirmation, Liabilities Subject to Compromise	5,278,612
Preconfirmation, Liabilities	6,334,156
Preconfirmation, Common Stock	2,145
Preconfirmation, Additional Paid-in Capital	3,056,307
Preconfirmation, Preferred and Common Stock Held in Treasury	(53,863)
Preconfirmation, Retained Earnings (Deficit)	(4,678,977)
Preconfirmation, Accumulated Other Comprehensive Income (Loss)	(1,706)
Preconfirmation, Stockholders' Equity	(1,676,094)
Preconfirmation, Liabilities and Stockholders' Equity	4,658,062
Stockholders’ equity (deficit)
Fresh-Start Adjustment, Increase (Decrease), Common Stock	(2,145)
Fresh-Start Adjustment, Increase (Decrease), Additional Paid-in Capital	(3,056,307)
Fresh-Start Adjustment, Increase (Decrease), Preferred and Common Stock Held in Treasury	53,863
Fresh-Start Adjustment, Increase (Decrease), Retained Earnings (Deficit)	7,146,693
Predecessor | Effect of Plan
Stockholders’ equity (deficit)
Fresh-Start Adjustment, Increase (Decrease), Common Stock	(2,145)
Fresh-Start Adjustment, Increase (Decrease), Additional Paid-in Capital	(3,056,307)
Successor
Postconfirmation
Postconfirmation, Cash and Cash Equivalents	200,487
Postconfirmation, Short-term Investments	111,451
Postconfirmation, Restricted Cash and Cash Equivalents, Current	81,563
Postconfirmation, Receivables, Net	165,522
Postconfirmation, Other Receivables	18,006
Postconfirmation, Inventories	138,332
Postconfirmation, Prepaid Royalties, Current	4,805
Postconfirmation, Deferred Income Tax Assets, Current	0
Postconfirmation, Coal Derivative Assets	2,180
Postconfirmation, Prepaid and Other Current Assets	97,178
Postconfirmation, Current Assets	819,524
Postconfirmation, Property and Equipment, Net	1,071,112
Postconfirmation, Prepaid Royalties, Noncurrent	0
Postconfirmation, Investments	102,626
Postconfirmation, Other Assets, Noncurrent	130,567
Postconfirmation, Other Assets	233,193
Postconfirmation, Assets	2,123,829
Postconfirmation, Accounts Payable	74,345
Postcomfirmation, Accrued expenses and other current liabilities	216,052
Postconfirmation, Current Maturities of Long-term Debt	6,662
Postconfirmation, Current Liabilities	297,059
Postconfirmation, Long-term Debt	353,272
Postconfirmation, Asset Retirement Obligations	334,129
Postconfirmation, Pension Obligations	48,281
Postconfirmation, Other Postretirement Obligations	111,959
Postconfirmation, Accrued Workers' Compensation	194,348
Postconfirmation, Deferred Income Tax Liabilities	0
Postconfirmation, Noncurrent Other Obligations	97,298
Postconfirmation, Liabilities Not Subject to Compromise	1,436,346
Postconfirmation, Liabilities Subject to Compromise	0
Postconfirmation, Liabilities	1,436,346
Postconfirmation, Common Stock	250
Postconfirmation, Additional Paid-in Capital	687,233
Postconfirmation, Preferred and Common Stock Held in Treasury	0
Postconfirmation, Retained Earnings (Deficit)	0
Postconfirmation, Accumulated Other Comprehensive Income (Loss)	0
Postconfirmation, Stockholders' Equity	687,483
Postconfirmation, Liabilities and Stockholders' Equity	2,123,829
Successor | Effect of Plan
Stockholders’ equity (deficit)
Fresh-Start Adjustment, Increase (Decrease), Common Stock	250
Fresh-Start Adjustment, Increase (Decrease), Additional Paid-in Capital	$ 687,233