Pension and Postretirement Plans - Components of Net Periodic Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.0	$ 0.0	$ 0.0
Interest cost on projected benefit obligation	20.0	19.4	19.3
Expected return on plan assets	(29.8)	(26.0)	(27.3)
Amortization of:
Prior service cost (benefit)	0.0	0.0	0.1
Actuarial loss	17.0	17.5	19.1
Pension settlement charges	0.1	4.0	0.0
Pension/postretirement cost (benefit)	7.3	14.9	11.2
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.6	0.2	0.3
Interest cost on projected benefit obligation	4.2	3.2	3.3
Expected return on plan assets	0.0	0.0	0.0
Amortization of:
Prior service cost (benefit)	(3.1)	(4.5)	(14.7)
Actuarial loss	4.1	4.7	4.9
Pension settlement charges	0.0	0.0	0.0
Pension/postretirement cost (benefit)	5.8	3.6	$ (6.2)
Cincinnati Plans [Member] | Pension Plan
Amortization of:
Pension settlement charges		$ 4.0
Hawaii Plans [Member] | Pension Plan
Amortization of:
Pension settlement charges	$ 0.1