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Operations and Significant Accounting Policies - Leases (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Lessee Disclosure [Abstract]    
Operating Lease Residual Value Guarantee Our largest operating lease is for the dragline at BNI Energy which includes a termination payment at the end of the lease term if we do not exercise our purchase option.  
Residual Value Guarantee $ 3.0  
Operating Lease Cost [Abstract]    
Operating Lease Cost 2.9  
Other Information [Abstract]    
Operating Cash Flows from Operating Leases 2.9  
Balance Sheet Information Related to Leases [Abstract]    
Operating Lease, Right-of-Use Asset [1] $ 34.0 $ 0.0
Other Non-Current Assets us-gaap:OperatingLeaseRightOfUseAsset  
Total Lease Obligations $ 34.0  
Operating Lease Liabilities $ 8.4 0.0
Other Current Liabilities us-gaap:OperatingLeaseLiabilityCurrent  
Operating Lease Liabilities $ 25.6 $ 0.0
Other Non-Current Liabilities us-gaap:OperatingLeaseLiabilityNoncurrent  
Vehicles and Equipment [Member]    
Lease Description [Line Items]    
Weighted Average Remaining Lease Term 4 years  
Weighted Average Discount Rate 3.60%  
Land and Other [Member]    
Lease Description [Line Items]    
Weighted Average Remaining Lease Term 29 years  
Weighted Average Discount Rate 4.50%  
[1] See Leases.