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Component: (Network and Table)
Network
2407402 - Disclosure - Asset Retirement Obligations Rollforward (Details)
(http://www.northwesternenergy.com/role/AssetRetirementObligationsRollforwardDetails)
Table(Implied)
Slicers (applies to each fact value in each table cell)
Reporting Entity [Axis]0000073088 (http://www.sec.gov/CIK)
Asset Retirement Obligation Disclosure [Abstract]Period [Axis]
2016-01-01 - 2016-12-31
2015-01-01 - 2015-12-31
Asset Retirement Obligation Disclosure [Abstract]
 
 
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
 
 
Liability at January 1,
35,532,000  
21,435,000  
Accretion expense
1,885,000  
1,437,000  
Liabilities incurred
164,000  
12,682,000  
Liabilities settled
0  
(22,000) 
Revision to cash flows
1,821,000  
0  
Liability at December 31,
39,402,000  
 
35,532,000