Operations and Significant Accounting Policies - Supplemental Statement of Cash Flows Information (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018
Supplemental Cash Flow Information [Abstract]
Cash Paid for Interest – Net of Amounts Capitalized		$ 53.5	$ 52.9
Recognition of Right-of-use Assets and Lease Liabilities	[1]	30.4	0.0
Noncash Investing and Financing Activities [Abstract]
Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment		6.3	(14.4)
Reclassification of Property, Plant and Equipment to Inventory	[2]	0.0	46.3
Capitalized Asset Retirement Costs		16.6	15.4
AFUDC–Equity		$ 1.9	$ 0.9

[1] See Leases.

[2] 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.