Long-Term Contractual Arrangements (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	$ 168.5
Unrecorded Unconditional Purchase Obligation, Due within Two Years	188.6
Unrecorded Unconditional Purchase Obligation, Due within Three Years	182.5
Unrecorded Unconditional Purchase Obligation, Due within Four Years	159.7
Unrecorded Unconditional Purchase Obligation, Due within Five Years	95.3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	391.7
Unrecorded Unconditional Purchase Obligation, Purchases	1,186.3
Operating Leases, Future Minimum Payments Due, Current	6.4
Operating Leases, Future Minimum Payments, Due in Two Years	11.5
Operating Leases, Future Minimum Payments, Due in Three Years	8.4
Operating Leases, Future Minimum Payments, Due in Four Years	7.3
Operating Leases, Future Minimum Payments Due Thereafter	12.2
Operating Leases, Future Minimum Payments Due	51.5
Operating Leases, Future Minimum Payments, Due in Five Years	5.7
Supply And Stranded Costs [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	16.3
Unrecorded Unconditional Purchase Obligation, Due within Two Years	28.7
Unrecorded Unconditional Purchase Obligation, Due within Three Years	33.0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	32.9
Unrecorded Unconditional Purchase Obligation, Due within Five Years	12.6
Unrecorded Unconditional Purchase Obligation, Due after Five Years	7.1
Unrecorded Unconditional Purchase Obligation, Purchases	130.6
Renewable Energy Supply Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	33.7
Unrecorded Unconditional Purchase Obligation, Due within Two Years	87.3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	86.6
Unrecorded Unconditional Purchase Obligation, Due within Four Years	87.9
Unrecorded Unconditional Purchase Obligation, Due within Five Years	52.1
Unrecorded Unconditional Purchase Obligation, Due after Five Years	252.8
Unrecorded Unconditional Purchase Obligation, Purchases	600.4
Transmission Support Commitments [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	1.9
Unrecorded Unconditional Purchase Obligation, Due within Two Years	3.7
Unrecorded Unconditional Purchase Obligation, Due within Three Years	3.6
Unrecorded Unconditional Purchase Obligation, Due within Four Years	3.5
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation, Purchases	12.7
Natural Gas Procurement Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	108.5
Unrecorded Unconditional Purchase Obligation, Due within Two Years	54.0
Unrecorded Unconditional Purchase Obligation, Due within Three Years	47.5
Unrecorded Unconditional Purchase Obligation, Due within Four Years	24.7
Unrecorded Unconditional Purchase Obligation, Due within Five Years	21.6
Unrecorded Unconditional Purchase Obligation, Due after Five Years	78.5
Unrecorded Unconditional Purchase Obligation, Purchases	334.8
Electric Interconnection Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	1.7
Unrecorded Unconditional Purchase Obligation, Due within Two Years	3.4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	3.4
Unrecorded Unconditional Purchase Obligation, Due within Four Years	3.4
Unrecorded Unconditional Purchase Obligation, Due within Five Years	3.3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	41.1
Unrecorded Unconditional Purchase Obligation, Purchases	$ 56.3