Employee Benefits Pension and postretirement benefit plan obligation disclosures (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 semimonthly_pay_periods	Dec. 31, 2013	Dec. 31, 2012
Change in plan assets [Roll Forward]
Fair value of plan assets at end of year	$ 737.2	$ 672.0
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	708.9
Change in benefit obligation [Roll Forward]
Benefit obligation at beginning of year	702.7	760.3
Service cost	11.8	13.9	13.2
Interest cost	33.4	30.6	31.1
Change in plan assumptions	109.1	(73.6)
Actuarial (gain)	(0.7)	(7.7)
Benefits paid	(22.3)	(20.8)
Benefit obligation at end of year	834.0	702.7	760.3
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	628.2	510.8
Actual gain on plan assets	40.4	80.2
Company contributions to the plan	43.9	58.0
Fair value of plan assets at end of year	690.2	628.2	510.8
Funded status of plan [Abstract]
Underfunded status (current and long-term)	(143.8)	(74.5)
Estimated benefit-plan payments for the next five years and subsequent five years aggregated [Abstract]
2015	67.3
2016	52.6
2017	47.2
2018	63.0
2019	53.6
2020 through 2024	265.0
Pension Plans, Defined Benefit, Qualified Only [Member]
Assumptions used to calculate benefit obligations [Abstract]
Discount rate used to calculate benefit obligation (in hundredths)	4.10%	4.90%
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)	7.00%	7.25%
Funded status of plan [Abstract]
Projected pension funding in the next fiscal year	44.0
Pension Plans, Defined Benefit, Nonqualified Only [Member]
Assumptions used to calculate benefit obligations [Abstract]
Discount rate used to calculate benefit obligation (in hundredths)	2.90%	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%	4.70%
Long-term return on assets used to calculate benefit obligation (in hundredths)	7.00%	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)	7.50%	8.00%
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	86.3	94.4
Service cost	0.6	0.7	0.7
Interest cost	3.8	3.7	3.8
Change in plan assumptions	4.9	(8.6)
Actuarial (gain)	(3.0)	(0.5)
Benefits paid	(1.8)	(3.4)
Benefit obligation at end of year	90.8	86.3	94.4
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	43.8	37.6
Actual gain on plan assets	2.9	5.8
Company contributions to the plan	2.1	3.8
Fair value of plan assets at end of year	47.0	43.8	37.6
Funded status of plan [Abstract]
Underfunded status (current and long-term)	(43.8)	(42.5)
Estimated benefit-plan payments for the next five years and subsequent five years aggregated [Abstract]
2015	4.3
2016	4.5
2017	4.6
2018	4.8
2019	5.0
2020 through 2024	$ 26.3