Employee Benefits Net pension and postretirement benefit cost and other disclosures (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Assumed health care cost trend rates used to calculate net benefit costs [Abstract]
Max gain or loss percentage for corridor approach	10.00%
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	$ 13.8	$ 11.8	$ 13.9
Interest cost	32.7	33.4	30.6
Expected return on plan assets	(47.7)	(43.6)	(38.1)
Prior service and other costs	0.1	0.6	1.1
Recognized net actuarial loss	21.9	15.4	28.9
Pension settlement costs	16.7	0.0	0.0
Accretion of regulatory liability	0.0	0.0	0.0
Net periodic cost	37.5	$ 17.6	$ 36.4
Assumed health care cost trend rates used to calculate net benefit costs [Abstract]
Estimated net benefit cost for next fiscal year	11.2
Estimated amortization of actuarial loss from AOCI for next fiscal year	$ 15.1
Pension Plans, Defined Benefit, Qualified Only [Member]
Assumptions used to calculate net pension and postretirement benefit costs [Abstract]
Discount rate used to calculate net benefit costs (in hundredths)	4.10%	4.90%	4.20%
Rate of increase in compensation used to calculate net benefit costs (in hundredths)	5.50%	5.50%	5.50%
Long-term return on assets used to calculate net benefit costs (in hundredths)	7.00%	7.25%	7.25%
Pension Plans, Defined Benefit, Nonqualified Only [Member]
Assumptions used to calculate net pension and postretirement benefit costs [Abstract]
Discount rate used to calculate net benefit costs (in hundredths)	2.90%	3.30%	2.40%
Rate of increase in compensation used to calculate net benefit costs (in hundredths)	5.50%	5.50%	5.50%
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service cost	$ 0.6	$ 0.6	$ 0.7
Interest cost	3.5	3.8	3.7
Expected return on plan assets	(3.2)	(3.0)	(2.6)
Prior service and other costs	0.0	0.0	0.0
Recognized net actuarial loss	1.2	0.7	3.0
Pension settlement costs	0.0	0.0	0.0
Accretion of regulatory liability	1.0	0.8	0.5
Net periodic cost	$ 3.1	$ 2.9	$ 5.3
Assumptions used to calculate net pension and postretirement benefit costs [Abstract]
Discount rate used to calculate net benefit costs (in hundredths)	4.00%	4.70%	4.00%
Long-term return on assets used to calculate net benefit costs (in hundredths)	7.00%	7.25%	7.25%
Assumed health care cost trend rates used to calculate net benefit costs [Abstract]
Health-care inflation rate used to calculate net benefit costs (in hundredths)	7.50%	8.00%	8.50%
Ultimate health care cost trend rate used to calculate net benefit costs (in hundredths)	4.50%	4.50%	4.50%
Year that rate reaches ultimate trend rate, used to calculate net benefit costs	2021	2021	2021
Estimated net benefit cost for next fiscal year	$ 2.1
Estimated amortization of actuarial loss from AOCI for next fiscal year	0.9
Estimated amortization of prior service cost from AOCI for next fiscal year	0.1
Effect of 1% increase in the health care inflation rate on the yearly service and interest cost	0.0
Effect of 1% increase in the health care inflation rate on the accumulated post-retirement benefit obligation	0.5
Effect of 1% decrease in the health care inflation rate on the yearly service and interest cost	0.0
Effect of 1% decrease in the health care inflation rate on the accumulated post-retirement benefit obligation	$ (0.5)