Operations and Significant Accounting Policies - Supplemental Statement of Cash Flows Information (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018
Supplemental Cash Flow Information [Abstract]
Cash Paid for Interest – Net of Amounts Capitalized		$ 33.8	$ 33.0
Noncash Investing and Financing Activities [Abstract]
Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment		21.1	(39.4)
Reclassification of Property, Plant and Equipment to Inventory	[1]	0.0	46.3
Recognition of Right-of-use Assets and Lease Liabilities	[2]	31.2	0.0
Capitalized Asset Retirement Costs		1.4	20.8
AFUDC–Equity		$ 1.3	$ 0.5

[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.