Employee Benefits Pension and postretirement benefit plan obligation disclosures (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) semimonthly_pay_periods	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 737.2
Fair value of plan assets at end of year	$ 705.2	$ 737.2
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 | semimonthly_pay_periods	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	$ 646.6
Change in benefit obligation [Roll Forward]
Benefit obligation at beginning of year	834.0	702.7
Service cost	13.8	11.8	$ 13.9
Interest cost	32.7	33.4	30.6
Plan and assumption changes	(53.7)	109.1
Actuarial (gain) loss	7.0	(0.7)
Benefits paid	(73.8)	(22.3)
Benefit obligation at end of year	760.0	834.0	702.7
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	690.2	628.2
Actual gain (loss) on plan assets	(15.0)	40.4
Company contributions to the plan	71.8	43.9
Fair value of plan assets at end of year	673.2	690.2	628.2
Funded status of plan [Abstract]
Underfunded status (current and long-term)	(86.8)	$ (143.8)
Estimated benefit-plan payments for the next five years and subsequent five years aggregated [Abstract]
2016	50.5
2017	50.1
2018	63.6
2019	52.9
2020	52.8
2021 through 2025	$ 264.1
Pension Plans, Defined Benefit, Qualified Only [Member]
Assumptions used to calculate benefit obligations [Abstract]
Discount rate used to calculate benefit obligation (in hundredths)	4.50%	4.10%
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%	7.00%
Funded status of plan [Abstract]
Projected pension funding in the next fiscal year	$ 18.1
Pension Plans, Defined Benefit, Nonqualified Only [Member]
Assumptions used to calculate benefit obligations [Abstract]
Discount rate used to calculate benefit obligation (in hundredths)	3.10%	2.90%
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%
Assumptions used to calculate benefit obligations [Abstract]
Discount rate used to calculate benefit obligation (in hundredths)	4.30%	4.00%
Long-term return on assets used to calculate benefit obligation (in hundredths)	7.00%	7.00%
Assumed health care cost trend rates used to calculate benefit obligation [Abstract]
Health-care inflation rate used to calculate benefit obligation (in hundredths)	7.00%	7.50%
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	$ 90.8	$ 86.3
Service cost	0.6	0.6	0.7
Interest cost	3.5	3.8	3.7
Plan and assumption changes	(4.4)	4.9
Actuarial (gain) loss	(1.6)	(3.0)
Benefits paid	(2.9)	(1.8)
Benefit obligation at end of year	86.0	90.8	86.3
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	47.0	43.8
Actual gain (loss) on plan assets	(0.9)	2.9
Company contributions to the plan	0.9	2.1
Fair value of plan assets at end of year	44.1	47.0	$ 43.8
Funded status of plan [Abstract]
Underfunded status (current and long-term)	(41.9)	$ (43.8)
Estimated benefit-plan payments for the next five years and subsequent five years aggregated [Abstract]
2016	4.5
2017	4.5
2018	4.7
2019	4.8
2020	5.0
2021 through 2025	$ 25.9