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Operations and Significant Accounting Policies - Supplemental Statement of Cash Flow Information - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Supplemental Cash Flow Information [Abstract]      
Cash Paid During the Period for Interest – Net of Amounts Capitalized $ 62.0 $ 63.5 $ 66.0
Recognition of Right-of-use Assets and Lease Liabilities [1] 0.0 28.7 0.0
Noncash Investing and Financing Activities [Abstract]      
Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment (67.0) 33.9 (0.1)
Reclassification of Property, Plant and Equipment to Inventory [2] 0.0 0.0 46.3
Costs Incurred, Capitalized Asset Retirement Obligation Costs 4.1 20.7 14.2
AFUDC–Equity $ 1.9 $ 2.3 $ 1.2
[1] Amount of the right-of-use asset and lease liability recognized with the adoption of an accounting standards update for leases.
[2] In 2018, Montana-Dakota Utilities exercised its option to purchase the Thunder Spirit II wind energy facility upon completion, resulting in a reclassification from Property, Plant and Equipment – Net to Inventories – Net for project costs incurred in the prior year. On the Consolidated Statement of Cash Flows, the sale of the wind energy facility in 2018 resulted in Operating Activities – Inventories increasing by $46.3 million in 2018 due to the project costs incurred in the prior year.