Employee Benefits Pension and postretirement benefit plan disclosures (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 semimonthly_pay_periods	Dec. 31, 2011	Dec. 31, 2010
Change in plan assets [Roll Forward]
Fair value of plan assets - ending balance	$ 548.4	$ 423.9
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of consecutive semimonthly pay periods of highest earnings, on which pension plan benefits are based	72
Number of final years of employment, during which the highest 72 consecutive semimonthly earnings periods for pension plan benefits are determined	10 years
Accumulated benefit obligation	623.4
Change in benefit obligation [Roll Forward]
Benefit obligation - beginning balance	646.9	492.4
Service cost	13.2	9.3	9.3
Interest cost	31.1	28.5	28.7
Change in plan assumptions	69.0	122.3
Actuarial loss	18.2	13.2
Benefits paid	(18.1)	(18.8)
Benefit obligation - ending balance	760.3	646.9	492.4
Change in plan assets [Roll Forward]
Fair value of plan assets - beginning balance	390.0	359.9
Actual gain on plan assets	59.3	2.5
Company contributions to the plan	79.6	46.4
Fair value of plan assets - ending balance	510.8	390.0	359.9
Funded status of plan [Abstract]
Underfunded status (current and long-term)	(249.5)	(256.9)
Estimated benefit-plan payments for the next five years and subsequent five years aggregated [Abstract]
2013	24.7
2014	26.0
2015	33.0
2016	28.7
2017	29.0
2018 through 2022	183.5
Components of pension and postretirement benefits expense [Abstract]
Service cost	13.2	9.3	9.3
Interest cost	31.1	28.5	28.7
Expected return on plan assets	(31.8)	(26.0)	(24.4)
Prior-service and other costs	1.1	1.1	1.1
Recognized net actuarial loss	24.9	12.1	8.2
Cutailment charges	0	0.8	2.4
Accretion of regulatory liability	0	0	0
Periodic expense	38.5	25.8	25.3
Pension Plans, Defined Benefit, Qualified Only [Member]
Assumptions used to calculate benefit obligations [Abstract]
Discount rate used to calculate benefit obligation (in hundredths)	4.20%	4.80%
Rate of increase in compensation used to calculate benefit obligation (in hundredths)	5.50%	5.50%
Long-term return on assets used to calculate benefit obligation (in hundredths)	7.25%	7.25%
Funded status of plan [Abstract]
Projected pension funding in the next fiscal year	57.7
Pension Plans, Defined Benefit, Nonqualified Only [Member]
Assumptions used to calculate benefit obligations [Abstract]
Discount rate used to calculate benefit obligation (in hundredths)	2.40%	3.30%
Rate of increase in compensation used to calculate benefit obligation (in hundredths)	5.50%	5.50%
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Percent of 1992 contribution, to which the Company's portion of health-care benefits is generally limited for employees who retired after January 1, 1993	170.00%
Period of time over which the transition obligation was amortized that began in 1992 and ended in 2012 (in years)	20 years
Assumptions used to calculate benefit obligations [Abstract]
Discount rate used to calculate benefit obligation (in hundredths)	4.00%	4.40%
Long-term return on assets used to calculate benefit obligation (in hundredths)	7.25%	7.25%
Assumed health care cost trend rates used to calculate benefit obligation [Abstract]
Health-care inflation rate used to calculate benefit obligation (in hundredths)	8.50%	8.00%
Ultimate health care cost trend rate used to calculate benefit obligation (in hundredths)	4.50%	5.00%
Year that rate reaches ultimate trend rate, used to calculate benefit obligation	2021	2014
Change in benefit obligation [Roll Forward]
Benefit obligation - beginning balance	88.7	77.9
Service cost	0.7	0.5	0.6
Interest cost	3.8	4.2	4.4
Change in plan assumptions	7.3	14.7
Actuarial loss	(1.0)	(0.9)
Benefits paid	(5.1)	(7.7)
Benefit obligation - ending balance	94.4	88.7	77.9
Change in plan assets [Roll Forward]
Fair value of plan assets - beginning balance	34.0	38.4
Actual gain on plan assets	4.6	(0.7)
Company contributions to the plan	4.1	4.0
Fair value of plan assets - ending balance	37.6	34.0	38.4
Funded status of plan [Abstract]
Underfunded status (current and long-term)	(56.8)	(54.7)
Estimated benefit-plan payments for the next five years and subsequent five years aggregated [Abstract]
2013	4.9
2014	5.1
2015	5.2
2016	5.3
2017	5.4
2018 through 2022	28.3
Components of pension and postretirement benefits expense [Abstract]
Service cost	0.7	0.5	0.6
Interest cost	3.8	4.2	4.4
Expected return on plan assets	(2.3)	(2.6)	(2.4)
Prior-service and other costs	1.6	1.9	1.9
Recognized net actuarial loss	2.5	0.3	0.6
Cutailment charges	0	0	0.3
Accretion of regulatory liability	0.8	0.8	0.8
Periodic expense	$ 7.1	$ 5.1	$ 6.2