Operations and Significant Accounting Policies - Leases (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended
		Sep. 30, 2019	Sep. 30, 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	$ 3.0
Operating Lease Cost [Abstract]
Operating Lease Cost		2.0	6.7
Other Information [Abstract]
Operating Cash Flows from Operating Leases		2.0	6.7
Balance Sheet Information Related to Leases [Abstract]
Operating Lease, Right-of-Use Asset	[1]	$ 30.4	$ 30.4	$ 0.0
Other Non-Current Assets		us-gaap:OperatingLeaseRightOfUseAsset	us-gaap:OperatingLeaseRightOfUseAsset
Total Lease Obligations		$ 30.4	$ 30.4
Operating Lease Liabilities	[2]	$ 7.1	$ 7.1	0.0
Other Current Liabilities		us-gaap:OperatingLeaseLiabilityCurrent	us-gaap:OperatingLeaseLiabilityCurrent
Operating Lease Liabilities		$ 23.3	$ 23.3	$ 0.0
Other Non-Current Liabilities		us-gaap:OperatingLeaseLiabilityNoncurrent	us-gaap:OperatingLeaseLiabilityNoncurrent
Vehicles and Equipment [Member]
Lease Description [Line Items]
Weighted Average Remaining Lease Term		4 years	4 years
Weighted Average Discount Rate		3.70%	3.70%
Land and Other [Member]
Lease Description [Line Items]
Weighted Average Remaining Lease Term		28 years	28 years
Weighted Average Discount Rate		4.10%	4.10%

[1] See Leases.

[2] See Leases.