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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.

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- Definition Description of terms and conditions of residual value guarantee provided by lessee on operating lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980 + Details
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- Definition Amount of single lease cost, calculated by allocation of remaining cost of lease over remaining lease term. Includes, but is not limited to, single lease cost, after impairment of right-of-use asset, calculated by amortization of remaining right-of-use asset and accretion of lease liability. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 55 -Paragraph 53 -URI http://asc.fasb.org/extlink&oid=77888399&loc=SL77918982-209971 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918686-209980 + Details
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