CONSOLIDATED STATEMENTS OF EQUITY (Cloud Peak Energy Inc. and Subs - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock Outstanding	Treasury Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss) [Member]
Balances at Dec. 31, 2014	$ 1,087,834	$ 610	$ (6,243)	$ 568,022	$ 536,744	$ (11,299)
Balances (in shares) at Dec. 31, 2014		61,022	432
Increase (Decrease in Stockholders' Equity
Net income (loss)	(204,900)	$ 0	$ 0	0	(204,900)	0
Postretirement benefit adjustment, net of tax	(1,652)	0	0	0	0	(1,652)
Write-off of excess tax benefits related to equity-based compensation	(415)	0	0	(415)	0	0
Employee stock purchases	586	$ 1	$ 0	585	0	0
Employee stock purchases (in shares)		136	0
Equity-based compensation expense	6,935	$ 0	$ 0	6,935	0	0
Restricted stock issuance, net of forfeitures	0	$ 1	$ 0	(1)	0	0
Restricted stock issuance, net of forfeitures (in shares)		56	1
Employee common stock withheld to cover withholding taxes	(507)	$ 0	$ (255)	(252)	0	0
Employee common stock withheld to cover withholding taxes (in shares)		(44)	44
Balances at Dec. 31, 2015	887,881	$ 612	$ (6,498)	574,874	331,844	(12,951)
Balances (in shares) at Dec. 31, 2015		61,170	477
Increase (Decrease in Stockholders' Equity
Net income (loss)	21,841	$ 0	$ 0	0	21,841	0
Postretirement benefit adjustment, net of tax	34,835	0	0	0	0	34,835
Employee stock purchases	458	$ 1	$ 0	457	0	0
Employee stock purchases (in shares)		134	0
Equity-based compensation expense	6,845	$ 0	$ 0	6,845	0	0
Restricted stock issuance, net of forfeitures	0	$ 2	$ 0	(2)	0	0
Restricted stock issuance, net of forfeitures (in shares)		230	0
Employee common stock withheld to cover withholding taxes	(199)	$ 0	$ 0	(199)	0	0
Employee common stock withheld to cover withholding taxes (in shares)		(69)	0
Balances at Dec. 31, 2016	951,661	$ 615	$ (6,498)	581,975	353,685	21,884
Balances (in shares) at Dec. 31, 2016		61,465	477
Increase (Decrease in Stockholders' Equity
Net income (loss)	(6,639)	$ 0	$ 0	0	(6,639)	0
Postretirement benefit adjustment, net of tax	(8,077)	0	0	0	0	(8,077)
Equity-based compensation expense	6,803	0	0	6,803	0	0
Restricted stock issuance, net of forfeitures	(10)	$ 3	$ 0	(13)	0	0
Restricted stock issuance, net of forfeitures (in shares)		268	0
Employee common stock withheld to cover withholding taxes	(289)	$ (1)	$ 0	(288)	0	0
Employee common stock withheld to cover withholding taxes (in shares)		(66)	0
Equity offering value	64,360	$ 135	$ 0	64,225	0	0
Equity offering in Shares		13,500	0
Balances at Dec. 31, 2017	$ 1,007,809	$ 752	$ (6,498)	$ 652,702	$ 347,046	$ 13,807
Balances (in shares) at Dec. 31, 2017		75,167	477