Employee Benefits Pension and postretirement benefit plan disclosures (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013 semimonthly_pay_periods	Dec. 31, 2012	Dec. 31, 2011
Change in plan assets [Roll Forward]
Fair value of plan assets at end of year	$ 672.0	$ 548.4
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	597.8
Change in benefit obligation [Roll Forward]
Benefit obligation at beginning of year	760.3	646.9
Service cost	13.9	13.2	9.3
Interest cost	30.6	31.1	28.5
Change in plan assumptions	(73.6)	69.0
Actuarial (gain) loss	(7.7)	18.2
Benefits paid	(20.8)	(18.1)
Benefit obligation at end of year	702.7	760.3	646.9
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	510.8	390.0
Actual gain on plan assets	80.2	59.3
Company contributions to the plan	58.0	79.6
Fair value of plan assets at end of year	628.2	510.8	390.0
Funded status of plan [Abstract]
Underfunded status (current and long-term)	(74.5)	(249.5)
Estimated benefit-plan payments for the next five years and subsequent five years aggregated [Abstract]
2014	26.7
2015	35.9
2016	27.9
2017	28.7
2018	31.1
2019 through 2023	195.7
Components of pension and postretirement benefits expense [Abstract]
Service cost	13.9	13.2	9.3
Interest cost	30.6	31.1	28.5
Expected return on plan assets	(38.1)	(31.8)	(26.0)
Prior service and other costs	1.1	1.1	1.1
Recognized net actuarial loss	28.9	24.9	12.1
Curtailment charges	0	0	0.8
Accretion of regulatory liability	0	0	0
Net periodic cost	36.4	38.5	25.8
Pension Plans, Defined Benefit, Qualified Only [Member]
Assumptions used to calculate benefit obligations [Abstract]
Discount rate used to calculate benefit obligation (in hundredths)	4.90%	4.20%
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	18.0
Pension Plans, Defined Benefit, Nonqualified Only [Member]
Assumptions used to calculate benefit obligations [Abstract]
Discount rate used to calculate benefit obligation (in hundredths)	3.30%	2.40%
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)	4.70%	4.00%
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)	8.00%	8.50%
Ultimate health care cost trend rate used to calculate benefit obligation (in hundredths)	4.50%	4.50%
Year that rate reaches ultimate trend rate, used to calculate benefit obligation	2021	2021
Change in benefit obligation [Roll Forward]
Benefit obligation at beginning of year	94.4	88.7
Service cost	0.7	0.7	0.5
Interest cost	3.7	3.8	4.2
Change in plan assumptions	(8.6)	7.3
Actuarial (gain) loss	(0.5)	(1.0)
Benefits paid	(3.4)	(5.1)
Benefit obligation at end of year	86.3	94.4	88.7
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	37.6	34.0
Actual gain on plan assets	5.8	4.6
Company contributions to the plan	3.8	4.1
Fair value of plan assets at end of year	43.8	37.6	34.0
Funded status of plan [Abstract]
Underfunded status (current and long-term)	(42.5)	(56.8)
Estimated benefit-plan payments for the next five years and subsequent five years aggregated [Abstract]
2014	4.6
2015	4.8
2016	4.9
2017	5.1
2018	5.2
2019 through 2023	27.4
Components of pension and postretirement benefits expense [Abstract]
Service cost	0.7	0.7	0.5
Interest cost	3.7	3.8	4.2
Expected return on plan assets	(2.6)	(2.3)	(2.6)
Prior service and other costs	0	1.6	1.9
Recognized net actuarial loss	3.0	2.5	0.3
Curtailment charges	0	0	0
Accretion of regulatory liability	0.5	0.3	0.5
Net periodic cost	$ 5.3	$ 6.6	$ 4.8