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Operations and Significant Accounting Policies - Supplemental Statement of Cash Flows Information (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Supplemental Cash Flow Information [Abstract]    
Cash Paid for Interest – Net of Amounts Capitalized $ 19.7 $ 19.3
Noncash Investing and Financing Activities [Abstract]    
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment (1.1) (48.1)
Reclassification of Property, Plant and Equipment to Inventory [1] 0.0 46.9
Recognition of Right-of-use Assets and Lease Liabilities [2] 34.0 0.0
Capitalized Asset Retirement Costs 1.6 0.8
AFUDC–Equity $ 0.6 $ 0.3
[1] In February 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.
[2] See Leases.