Retirement Plans & Other Postretirement Benefits (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Net periodic benefit costs [Abstract]
Portion of benefit costs capitalized as Utility plant		$ 3.9	$ 4.3	$ 4.5
Plan Assets [Roll Forward]
Plan assets at fair value, beginning of period [Roll Forward]		240.3
Fair value of plan assets, end of period		261.0	240.3
Target allocation percentages [Abstract]
Guaranteed Annuity Contract - estimate of undiscounted funds necessary to satisfy John Hancock's remaining obligation		3.4	3.1
Guaranteed Annuity Contract - percent of composite investment return, net of manger fees and other charges (in hundredths)		5.26%	5.37%
Fair value of plan assets		261.0	240.3
Qualified Plans
Fair value of plan assets, end of period		261.0	240.3
Payments expected to be made to SERP participants in the next fiscal year		0.7
Reconciliation of amounts in Accumulated other comprehensive income and Regulatory assets
Less: Regulatory asset deferral		226.0	189.4
Fair Value of guaranteed annuity contract [Roll Forward]
Fair value, beginning of year		3.7	3.6
Unrealized gains related to investments still held at reporting date		0.2	0.2
Purchases, sales and settlements, net		(0.1)	(0.1)
Multiemployer Benefit Plan [Abstract]
Expense for the Multiemployer Benefit plan		18.3	10.0	8.8
Number of unions the Company contributed to on behalf of employees		260
Average contributions to each union		less than $0.1 million
Largest union multiemployer plan contribution		1.1
Number of multiemployer plans to which the Company made contributions (in number of plans)		17
Defined Contribution Plan [Abstract]
Contributions to Defined Contribution Plan		6.2	6.6	4.6
Domestic equities and equity funds [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		121.2	115.4
Target allocation percentages [Abstract]
Fair value of plan assets		121.2	115.4
Qualified Plans
Fair value of plan assets, end of period		121.2	115.4
International equities and equity funds [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		28.6	29.7
Target allocation percentages [Abstract]
Fair value of plan assets		28.6	29.7
Qualified Plans
Fair value of plan assets, end of period		28.6	29.7
Domestic bonds and bond funds [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		77.3	66.6
Target allocation percentages [Abstract]
Fair value of plan assets		77.3	66.6
Qualified Plans
Fair value of plan assets, end of period		77.3	66.6
Inflation protected security fund [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		11.8	9.2
Target allocation percentages [Abstract]
Fair value of plan assets		11.8	9.2
Qualified Plans
Fair value of plan assets, end of period		11.8	9.2
Real estate, commodities and other [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		22.1	19.4
Target allocation percentages [Abstract]
Fair value of plan assets		22.1	19.4
Qualified Plans
Fair value of plan assets, end of period		22.1	19.4
Fair Value, Inputs, Level 1 [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		129.0	126.2
Target allocation percentages [Abstract]
Fair value of plan assets		129.0	126.2
Qualified Plans
Fair value of plan assets, end of period		129.0	126.2
Fair Value, Inputs, Level 1 [Member] | Domestic equities and equity funds [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		54.7	54.6
Target allocation percentages [Abstract]
Fair value of plan assets		54.7	54.6
Qualified Plans
Fair value of plan assets, end of period		54.7	54.6
Fair Value, Inputs, Level 1 [Member] | International equities and equity funds [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		28.6	29.7
Target allocation percentages [Abstract]
Fair value of plan assets		28.6	29.7
Qualified Plans
Fair value of plan assets, end of period		28.6	29.7
Fair Value, Inputs, Level 1 [Member] | Domestic bonds and bond funds [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		38.2	35.3
Target allocation percentages [Abstract]
Fair value of plan assets		38.2	35.3
Qualified Plans
Fair value of plan assets, end of period		38.2	35.3
Fair Value, Inputs, Level 1 [Member] | Inflation protected security fund [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		0	0
Target allocation percentages [Abstract]
Fair value of plan assets		0	0
Qualified Plans
Fair value of plan assets, end of period		0	0
Fair Value, Inputs, Level 1 [Member] | Real estate, commodities and other [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		7.5	6.6
Target allocation percentages [Abstract]
Fair value of plan assets		7.5	6.6
Qualified Plans
Fair value of plan assets, end of period		7.5	6.6
Fair Value, Inputs, Level 2 [Member]
Target allocation percentages [Abstract]
Portion of Common Collective Trust Funds comprised of equity funds (in hundredths)		52.00%	55.00%
Portion of Common Collective Trust Funds comprised of fixed income funds (in hundredths)		40.00%	37.00%
Fair value of Common Collective Trust Funds		128.2	110.4
Maximum number of days restriction for exchange of shares in Common Collective Trust Funds (in days)		31
Fair Value, Inputs, Level 2 [Member] | Domestic equities and equity funds [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		66.5	60.8
Target allocation percentages [Abstract]
Fair value of plan assets		66.5	60.8
Qualified Plans
Fair value of plan assets, end of period		66.5	60.8
Fair Value, Inputs, Level 2 [Member] | International equities and equity funds [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		0	0
Target allocation percentages [Abstract]
Fair value of plan assets		0	0
Qualified Plans
Fair value of plan assets, end of period		0	0
Fair Value, Inputs, Level 2 [Member] | Domestic bonds and bond funds [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		39.1	31.3
Target allocation percentages [Abstract]
Fair value of plan assets		39.1	31.3
Qualified Plans
Fair value of plan assets, end of period		39.1	31.3
Fair Value, Inputs, Level 2 [Member] | Inflation protected security fund [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		11.8	9.2
Target allocation percentages [Abstract]
Fair value of plan assets		11.8	9.2
Qualified Plans
Fair value of plan assets, end of period		11.8	9.2
Fair Value, Inputs, Level 2 [Member] | Real estate, commodities and other [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		10.8	9.1
Target allocation percentages [Abstract]
Fair value of plan assets		10.8	9.1
Qualified Plans
Fair value of plan assets, end of period		10.8	9.1
Fair Value, Inputs, Level 3 [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		3.8	3.7
Target allocation percentages [Abstract]
Fair value of plan assets		3.8	3.7
Qualified Plans
Fair value of plan assets, end of period		3.8	3.7
Fair Value, Inputs, Level 3 [Member] | Domestic equities and equity funds [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		0	0
Target allocation percentages [Abstract]
Fair value of plan assets		0	0
Qualified Plans
Fair value of plan assets, end of period		0	0
Fair Value, Inputs, Level 3 [Member] | International equities and equity funds [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		0	0
Target allocation percentages [Abstract]
Fair value of plan assets		0	0
Qualified Plans
Fair value of plan assets, end of period		0	0
Fair Value, Inputs, Level 3 [Member] | Domestic bonds and bond funds [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		0	0
Target allocation percentages [Abstract]
Fair value of plan assets		0	0
Qualified Plans
Fair value of plan assets, end of period		0	0
Fair Value, Inputs, Level 3 [Member] | Inflation protected security fund [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		0	0
Target allocation percentages [Abstract]
Fair value of plan assets		0	0
Qualified Plans
Fair value of plan assets, end of period		0	0
Fair Value, Inputs, Level 3 [Member] | Real estate, commodities and other [Member]
Plan Assets [Roll Forward]
Fair value of plan assets, end of period		3.8	3.7
Target allocation percentages [Abstract]
Fair value of plan assets		3.8	3.7
Qualified Plans
Fair value of plan assets, end of period		3.8	3.7
Other Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of qualified defined benefit pension plans		3
Net periodic benefit costs [Abstract]
Service cost		6.9	6.3	6.3
Interest cost		15.9	15.9	15.8
Expected return on plan assets		(21.2)	(18.4)	(16.4)
Amortization of prior service cost (benefit)		1.7	1.6	1.7
Amortization of actuarial loss (gain)		3.8	3.2	2.2
Amortization of transitional obligation		0	0	0
Net periodic benefit cost		7.1	8.6	9.6
Discount rate (in hundredths)	4.82%	5.50%	6.00%	6.25%
Rate of compensation increase (in hundredths)		3.50%	3.50%	3.75%
Expected return on plan assets (in hundredths)		8.00%	8.00%	8.25%
Assumptions used to calculate benefit obligations [Abstract]
Discount rate (in hundredths)		4.82%	5.50%
Rate of compensation increase (in hundredths)		3.50%	3.50%
Benefit Obligation [Roll Forward]
Benefit obligation, beginning of period	329.2	297.3	271.5
Service cost - benefits earned during the period		6.9	6.3	6.3
Interest cost on projected benefit obligation		15.9	15.9	15.8
Plan participants' contributions		0	0
Plan amendments		0	0.8
Actuarial loss (gain)		23.1	21.3
Medicare subsidy receipts		0	0
Benefit payments		(14.0)	(18.5)
Benefit obligation, end of period		329.2	297.3	271.5
Accumulated benefit obligation for all defined benefit pension plans		310.9	280.5
Plan Assets [Roll Forward]
Plan assets at fair value, beginning of period [Roll Forward]	261.0	237.2	211.1
Actual return on plan assets		2.1	26.8
Employer contributions		35.7	17.8
Plan participants' contributions		0	0
Benefit payments		(14.0)	(18.5)
Fair value of plan assets, end of period		261.0	237.2	211.1
Target allocation percentages [Abstract]
Target percentage of investments in equity instruments (in hundredths)		60.00%
Target percentage of investments in debt instruments (in hundredths)		35.00%
Target percentage of investments in other investments, including real estate (in hundredths)		5.00%
Fair value of plan assets		261.0	237.2	211.1
Qualified Plans
Benefit obligation, end of period		(314.7)	(285.5)
Fair value of plan assets, end of period		261.0	237.2	211.1
Funded Status of Qualified Plans, end of period		(53.7)	(48.3)
Benefit obligation of SERP Plan, end of period		(14.5)	(11.8)
Total funded status, end of period		(68.2)	(60.1)
Accrued liabilities		0.7	0.7
Deferred credits and other liabilities		67.5	59.4
Contributions expected to be made to pension plan trusts in the next fiscal year		15.0
Estimated future benefit payments [Abstract]
Expected future benefit payments, year one		16.4
Expected future benefit payments, year two		17.3
Expected future benefit payments, year three		17.9
Expected future benefit payments, year four		18.7
Expected future benefit payments, year five		19.8
Expected future benefit payments, years six through ten		125.2
Prior service costs [Roll Forward]
Balance, beginning of year	5.4	7.1	7.9	9.5
Amounts arising during the period		0	0.8	0.1
Reclassification to benefit costs		(1.7)	(1.6)	(1.7)
Balance, end of year		5.4	7.1	7.9
Net gain or loss [Roll Forward]
Balance, beginning of year	116.6	78.2	68.5	90.9
Amounts arising during the period		42.2	12.9	(20.2)
Reclassification to benefit costs		(3.8)	(3.2)	(2.2)
Balance, end of year		116.6	78.2	68.5
Reconciliation of amounts in Accumulated other comprehensive income and Regulatory assets
Prior service cost		5.4	7.1	7.9
Unamortized actuarial gain/(loss)		116.6	78.2	68.5
Sub total		122.0	85.3
Less: Regulatory asset deferral		(115.9)	(81.0)
AOCI before taxes		6.1	4.3
Amounts that will be amortized from Accumulated Other Comprehensive Income (Loss) in next year [Abstract]
Prior service cost expected to be amortized in next year		1.6
Actuarial gain/loss expected to be amortized in next year		6.8
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of other postretirement benefit plans		3
Net periodic benefit costs [Abstract]
Service cost		0.5	0.5	0.5
Interest cost		4.3	4.6	4.4
Expected return on plan assets		0	(0.4)	(0.3)
Amortization of prior service cost (benefit)		(0.8)	(0.8)	(0.8)
Amortization of actuarial loss (gain)		0.6	0.5	0.4
Amortization of transitional obligation		1.1	1.2	1.1
Net periodic benefit cost		5.7	5.6	5.3
Discount rate (in hundredths)		5.50%	6.00%	6.25%
Expected return on plan assets (in hundredths)		8.00%	8.00%	8.25%
Expected increase in Consumer Price Index (in hundredths)		3.00%	3.00%	3.50%
Assumptions used to calculate benefit obligations [Abstract]
Discount rate (in hundredths)		4.78%	5.50%
Expected increase in Consumer Price Index (in hundredths)		2.75%	3.00%
Benefit Obligation [Roll Forward]
Benefit obligation, beginning of period		80.7	79.6
Service cost - benefits earned during the period		0.5	0.5	0.5
Interest cost on projected benefit obligation		4.3	4.6	4.4
Plan participants' contributions		1.9	1.7
Plan amendments		0	0
Actuarial loss (gain)		(0.5)	1.2
Medicare subsidy receipts		1.0	0.5
Benefit payments		(8.2)	(7.4)
Benefit obligation, end of period		79.7	80.7	79.6
Assumption of percentage increase in medical claims cost for next fiscal year (in hundredths)		8.00%
Ultimate trending increase of medical claims cost (in hundredths)		5.00%
Year that rate reaches ultimate trend rate		2018
Dollar impact of a one-percentage point change in assumed health care cost trend rates		2.5
Plan Assets [Roll Forward]
Plan assets at fair value, beginning of period [Roll Forward]		3.1	4.0
Actual return on plan assets		0.1	0.3
Employer contributions		3.1	4.5
Plan participants' contributions		1.9	1.7
Benefit payments		(8.2)	(7.4)
Fair value of plan assets, end of period		0	3.1	4.0
Target allocation percentages [Abstract]
Fair value of plan assets		0	3.1	4.0
Qualified Plans
Benefit obligation, end of period		(79.7)	(80.7)
Fair value of plan assets, end of period		0	3.1	4.0
Funded Status of Qualified Plans, end of period		(79.7)	(77.6)
Benefit obligation of SERP Plan, end of period		0	0
Total funded status, end of period		(79.7)	(77.6)
Accrued liabilities		5.1	4.6
Deferred credits and other liabilities		74.6	73.0
Other postretirement benefit payments to be made during the next fiscal year		5.1
Estimated future benefit payments [Abstract]
Expected future benefit payments, year one		7.5
Expected future benefit payments, year two		8.1
Expected future benefit payments, year three		8.7
Expected future benefit payments, year four		9.2
Expected future benefit payments, year five		9.8
Expected future benefit payments, years six through ten		58.1
Prior service costs [Roll Forward]
Balance, beginning of year		(2.0)	(2.8)	(3.7)
Amounts arising during the period		0	0	0.1
Reclassification to benefit costs		0.8	0.8	0.8
Balance, end of year		(1.2)	(2.0)	(2.8)
Net gain or loss [Roll Forward]
Balance, beginning of year		10.3	9.7	3.5
Amounts arising during the period		(0.6)	1.1	6.6
Reclassification to benefit costs		(0.6)	(0.5)	(0.4)
Balance, end of year		9.1	10.3	9.7
Transition obligation [Roll Forward]
Balance, beginning of year		3.8	5.0	6.2
Amounts arising during the period		0	0	(0.1)
Reclassification to benefit costs		(1.1)	(1.2)	(1.1)
Balance, end of year		2.7	3.8	5.0
Reconciliation of amounts in Accumulated other comprehensive income and Regulatory assets
Prior service cost		(1.2)	(2.0)	(2.8)
Unamortized actuarial gain/(loss)		9.1	10.3	9.7
Transition obligation		2.7	3.8	5.0
Sub total		10.6	12.1
Less: Regulatory asset deferral		(10.1)	(11.5)
AOCI before taxes		0.5	0.6
Amounts that will be amortized from Accumulated Other Comprehensive Income (Loss) in next year [Abstract]
Prior service cost expected to be amortized in next year		0.8
Actuarial gain/loss expected to be amortized in next year		0.5
Transition obligation expected to be amortized in next year		$ 1.1