XML 82 R69.htm IDEA: XBRL DOCUMENT v3.10.0.1
Commitments and Contingencies (Details 2)
Mar. 26, 2018
Item
Feb. 01, 2018
a
Sep. 20, 2017
Item
Aug. 22, 2017
Item
Jul. 17, 2017
Item
Oct. 21, 2016
Item
Mar. 15, 2016
Item
Nov. 25, 2015
Item
Sep. 15, 2015
Item
Antelope Mine [Member]                  
Commitment and Contingency [Line Items]                  
Acres related to estimated quantity of mineral rights under challenge | a   857              
California [Member]                  
Commitment and Contingency [Line Items]                  
Complaints Filed         3        
Northern Plains Resource Council Regulatory Challenge To Office Of Surface Minings Approval Process For Mine Plans Litigation [Member] | Antelope Mine [Member]                  
Commitment and Contingency [Line Items]                  
Number of different coal mines located in two different states for which approvals are challenged                 4
Challenged Mine Plan Modifications                 1
Northern Plains Resource Council Regulatory Challenge To Office Of Surface Minings Approval Process For Mine Plans Litigation [Member] | COLORADO [Member] | Antelope Mine [Member]                  
Commitment and Contingency [Line Items]                  
Number of different coal mines located in two different states for which approvals are challenged                 1
Northern Plains Resource Council Regulatory Challenge To Office Of Surface Minings Approval Process For Mine Plans Litigation [Member] | WYOMING [Member] | Antelope Mine [Member]                  
Commitment and Contingency [Line Items]                  
Number of different coal mines located in two different states for which approvals are challenged                 3
Wild Earth Guardians Regulatory Challenge to Office of Surface Minings Approval Process for Mine Plans Litigation [Member] | Antelope Mine [Member]                  
Commitment and Contingency [Line Items]                  
Administrative Record Documents           3      
Notices of Supplemental Authority     1 1          
Wild Earth Guardians Regulatory Challenge to Office of Surface Minings Approval Process for Mine Plans Litigation [Member] | WYOMING [Member] | Antelope Mine [Member]                  
Commitment and Contingency [Line Items]                  
Number of Mines included in OSM motion to sever complaint and transfer               2  
BLM Approval of Revised Resource Managment Plans [Member]                  
Commitment and Contingency [Line Items]                  
Lease Planning Areas             2    
Claims Filed 3