Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Business Segments [Line Items]
Operating Revenue	$ 263.8	$ 240.0
Fuel and Purchased Power Expense	86.5	77.1
Operating and Maintenance Expense	104.7	99.9
Depreciation Expense	28.2	24.6
Operating Income (Loss)	44.4	38.4
Interest Expense	(12.3)	(11.0)
Equity Earnings in ATC	5.2	4.6
Other Income (Expense)	2.7	0.7
Income (Loss) Before Income Taxes	40.0	32.7
Income Tax Expense (Benefit)	7.5	8.3
Net Income	32.5	24.4
Total Assets	3,251.3	2,906.6	3,253.4
Property, Plant and Equipment - Net	2,366.7	2,002.8	2,347.6
Accumulated Depreciation	1,179.8	1,099.0
Capital Additions	43.1	39.7
Regulated Operations [Member]
Business Segments [Line Items]
Operating Revenue	241.4	218.6
Fuel and Purchased Power Expense	86.5	77.1
Operating and Maintenance Expense	82.2	78.1
Depreciation Expense	26.8	23.2
Operating Income (Loss)	45.9	40.2
Interest Expense	(10.7)	(9.6)
Equity Earnings in ATC	5.2	4.6
Other Income (Expense)	1.1	0.8
Income (Loss) Before Income Taxes	41.5	36.0
Income Tax Expense (Benefit)	9.4	11.6
Net Income	32.1	24.4
Total Assets	2,967.9	2,586.4
Property, Plant and Equipment - Net	2,300.8	1,945.6
Accumulated Depreciation	1,121.8	1,045.9
Capital Additions	42.6	38.7
Investments and Other [Member]
Business Segments [Line Items]
Land in Minnesota (in acres)	6,000
Operating Revenue	22.4	21.4
Fuel and Purchased Power Expense	0	0
Operating and Maintenance Expense	22.5	21.8
Depreciation Expense	1.4	1.4
Operating Income (Loss)	(1.5)	(1.8)
Interest Expense	(1.6)	(1.4)
Equity Earnings in ATC	0	0
Other Income (Expense)	1.6	(0.1)
Income (Loss) Before Income Taxes	(1.5)	(3.3)
Income Tax Expense (Benefit)	(1.9)	(3.3)
Net Income	0.4	0
Total Assets	283.4	320.2
Property, Plant and Equipment - Net	65.9	57.2
Accumulated Depreciation	58.0	53.1
Capital Additions	$ 0.5	$ 1.0