Pension and Postretirement Benefit Costs (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) qualified_pension_plans	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) qualified_pension_plans	Jun. 30, 2018 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Voluntary Employees Beneficiary Association, Contribution Amount	$ 17.0		$ 17.0
Components of net periodic benefit cost
Net periodic cost	4.8	$ 4.6	9.7	$ 9.1
Other Defined Benefit Plan Information [Abstract]
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 20.0		$ 20.0
Number of Qualified Pension Plans | qualified_pension_plans	1		1
Defined benefit pension plans
Components of net periodic benefit cost
Service cost for benefits earned	$ 0.5	0.5	$ 1.0	1.1
Interest cost on projected benefit obligation	8.4	7.9	16.7	15.7
Expected return on plan assets	(7.8)	(10.7)	(15.6)	(21.4)
Net periodic cost	$ 1.1	(2.3)	$ 2.1	(4.6)
Other Defined Benefit Plan Information [Abstract]
Funding threshold	80.00%		80.00%
Postretirement benefit plans
Components of net periodic benefit cost
Service cost for benefits earned	$ 1.2	2.1	$ 2.4	4.1
Interest cost on projected benefit obligation	6.3	7.1	12.6	14.2
Expected return on plan assets	(0.1)	0.0	(0.2)	0.0
Amortization of prior service cost and net actuarial loss	(2.2)	0.0	(4.4)	0.0
Net periodic cost	$ 5.2	$ 9.2	$ 10.4	$ 18.3