Long-Term Debt and Capital Lease Obligations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 Megawatt	Dec. 31, 2013	Dec. 31, 2009
Long-term debt outstanding maturities and sinking fund requirements
Next year	$ 399.5
Year 2	27.4
Year 3	29.5
Year 4	281.1
Year 5	282.7
Thereafter	3,532.2
Total	4,552.4
Summary of capitalized leased facilities
Long-term power purchase commitment	140.3	140.3
Accumulated amortization	(98.3)	(92.5)
Total Leased Facilities	42.0	47.8
Future minimum lease payments under capital lease and present value of net minimum lease payments
Due next year	43.5
Due in two years	45.1
Due in three years	13.9
Due in four years	14.7
Due in five years	15.5
Thereafter	41.3
Total Minimum Lease Payments	174.0
Less: Estimated Executory Costs	(54.7)
Capital Leases Future Minimum Payments Due, Less Executory Costs	119.3
Less: Interest	(34.8)
Net Minimum Lease Payments	84.5
Less: Due Currently	(24.6)
Future minimum lease payments under our capital lease and the present value of our net minimum lease payments	59.9
Long-Term Debt and Capital Lease Obligations (Textuals) [Abstract]
Number of series of tax-exempt pollution control refunding bonds	2
Restriction under RCC	The RCC provides that we may not redeem, defease or purchase and our subsidiaries may not purchase any Junior Notes on or before May 15, 2037, unless, subject to certain limitations described in the RCC, during the 180 days prior to the date of redemption, defeasance or purchase, we have received a specified amount of proceeds from the sale of qualifying securities.
Period of Power purchase contract with an unaffiliated independent power producer	25 years
Power capacity from a gas-fired cogeneration facility under capital lease (MW)	236
Minimum energy requirement in gas-fired cogeneration facility	0
Power purchase contract expiration year	Dec. 31, 2022
Power purchase contract expected future renewable period	10 years
Total operating lease payment	34.9	33.7
Increase in regulatory asset due to minimum lease payment			78.5
Regulatory asset value at the end of life of contract	0
Total capital lease obligation	84.5	104.3
Capital lease obligation at the end of life of contract	0
Notes (secured, nonrecourse), 6.09% due 2030-2040 [Member]
Long-Term Debt and Capital Lease Obligations (Textuals) [Abstract]
Debt instrument interest rate stated percentage rate	6.09%	6.09%
Notes (unsecured), 6.20% due 2033 [Member]
Long-Term Debt and Capital Lease Obligations (Textuals) [Abstract]
Unsecured Long-term Debt, Noncurrent	200.0	200.0
Debt instrument interest rate stated percentage rate	6.20%	6.20%
Junior Notes (unsecured), 6.25% due 2067 [Member]
Long-Term Debt and Capital Lease Obligations (Textuals) [Abstract]
Unsecured Long-term Debt, Noncurrent	500.0	500.0
Interest Rate effective May 2017 for Junior Notes	the three-month LIBOR Rate plus 211.25 basis points and will reset quarterly
Debt instrument interest rate stated percentage rate	6.25%	6.25%
Parent Company [Member]
Long-term debt outstanding maturities and sinking fund requirements
Next year	0
Year 2	0
Year 3	0
Year 4	0
Year 5	0
Thereafter	700.0
Total	700.0
Wisconsin Electric [Member]
Long-Term Debt and Capital Lease Obligations (Textuals) [Abstract]
Unsecured Long-term Debt, Noncurrent	$ 147.0