Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Business Segments [Line Items]
Operating Revenue	$ 251.0	$ 248.8	$ 750.4	$ 705.2
Fuel and Purchased Power Expense	80.5	79.5	245.7	228.7
Operating and Maintenance Expense	102.7	98.7	311.2	294.8
Depreciation Expense	29.4	25.0	86.3	74.4
Operating Income (Loss)	38.4	45.6	107.2	107.3
Interest Expense	(12.7)	(12.3)	(37.8)	(33.4)
Equity Earnings in ATC	4.9	4.9	15.1	14.3
Other Income (Expense)	3.3	1.5	7.5	3.4
Income (Loss) Before Income Taxes	33.9	39.7	92.0	91.6
Income Tax Expense (Benefit)	8.7	10.3	20.3	23.4
Net Income (Loss)	25.2	29.4	71.7	68.2
Total Assets	3,463.9	3,138.1	3,463.9	3,138.1	3,253.4
Property, Plant and Equipment - Net	2,456.1	2,239.9	2,456.1	2,239.9	2,347.6
Accumulated Depreciation	1,225.3	1,146.7	1,225.3	1,146.7
Capital Additions			186.4	318.3
Regulated Operations [Member]
Business Segments [Line Items]
Operating Revenue	226.4	226.4	683.6	642.0
Fuel and Purchased Power Expense	80.5	79.5	245.7	228.7
Operating and Maintenance Expense	78.6	76.4	243.6	230.6
Depreciation Expense	27.9	23.5	81.8	70.1
Operating Income (Loss)	39.4	47.0	112.5	112.6
Interest Expense	(10.4)	(10.2)	(31.5)	(29.7)
Equity Earnings in ATC	4.9	4.9	15.1	14.3
Other Income (Expense)	1.2	1.5	3.4	3.5
Income (Loss) Before Income Taxes	35.1	43.2	99.5	100.7
Income Tax Expense (Benefit)	10.5	13.9	26.5	32.6
Net Income (Loss)	24.6	29.3	73.0	68.1
Total Assets	3,086.3	2,830.9	3,086.3	2,830.9
Property, Plant and Equipment - Net	2,384.1	2,180.8	2,384.1	2,180.8
Accumulated Depreciation	1,165.8	1,091.5	1,165.8	1,091.5
Capital Additions			177.6	312.6
Investments and Other [Member]
Business Segments [Line Items]
Land in Minnesota (in acres)	6,000		6,000
Operating Revenue	24.6	22.4	66.8	63.2
Fuel and Purchased Power Expense	0	0	0	0
Operating and Maintenance Expense	24.1	22.3	67.6	64.2
Depreciation Expense	1.5	1.5	4.5	4.3
Operating Income (Loss)	(1.0)	(1.4)	(5.3)	(5.3)
Interest Expense	(2.3)	(2.1)	(6.3)	(3.7)
Equity Earnings in ATC	0	0	0	0
Other Income (Expense)	2.1	0	4.1	(0.1)
Income (Loss) Before Income Taxes	(1.2)	(3.5)	(7.5)	(9.1)
Income Tax Expense (Benefit)	(1.8)	(3.6)	(6.2)	(9.2)
Net Income (Loss)	0.6	0.1	(1.3)	0.1
Total Assets	377.6	307.2	377.6	307.2
Property, Plant and Equipment - Net	72.0	59.1	72.0	59.1
Accumulated Depreciation	59.5	55.2	59.5	55.2
Capital Additions			$ 8.8	$ 5.7