Pension and Postretirement Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Capitalized portion of defined benefit contribution percent		14.00%		13.00%
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Payment for Pension and Other Postretirement Benefits			$ 2.8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 0.0	$ 0.0	0.0	$ 0.0
Other components of pension and postretirement benefit plans expense:
Interest cost on projected benefit obligation	4.1	4.9	8.3	9.7
Expected return on plan assets	(6.2)	(6.5)	(12.4)	(13.0)
Amortization of:
Prior service benefit	0.0	0.0	0.0	0.0
Actuarial loss	4.2	4.4	8.5	8.8
Total amortization	4.2	4.4	8.5	8.8
Pension / postretirement costs	2.1	2.8	4.4	5.5
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Estimated Future Employer Contributions in Current Fiscal Year	9.0		9.0
Payment for Pension and Other Postretirement Benefits			3.5
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	0.0	0.0	0.1	0.1
Other components of pension and postretirement benefit plans expense:
Interest cost on projected benefit obligation	0.8	0.8	1.6	1.6
Expected return on plan assets	0.0	0.0	0.0	0.0
Amortization of:
Prior service benefit	(0.8)	(1.1)	(1.6)	(2.2)
Actuarial loss	1.1	1.2	2.1	2.3
Total amortization	0.3	0.1	0.5	0.1
Pension / postretirement costs	1.1	$ 0.9	2.2	$ 1.8
Qualified Plan [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Estimated Future Employer Contributions in Current Fiscal Year	4.0		4.0
Nonqualified Plan [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Estimated Future Employer Contributions in Current Fiscal Year	$ 3.0		$ 3.0