Pension and Postretirement Plans - Benefit Obligation and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1,	$ 399.6
Fair value of plan assets at December 31,	488.2	$ 399.6
Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1,	489.2	505.6
Hawaiian Telcom opening balance sheet adjustment	184.1	0.0
Service cost	0.0	0.0	$ 0.0
Interest cost	20.0	19.4	19.3
Actuarial (gain) loss	(39.9)	28.1
Benefits paid	(43.0)	(38.6)
Retiree drug subsidy received	0.0	0.0
Transfer of special death benefit	0.0	14.0
Settlements	(3.6)	(11.3)
Other	0.0	0.0
Benefit obligation at December 31,	606.8	489.2	505.6
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1,	392.1	372.3
Hawaiian Telcom opening balance sheet adjustment	163.0	0.0
Actual (loss) return on plan assets	(37.7)	64.8
Employer contributions	11.6	4.6
Retiree drug subsidy received	0.0	0.0
Benefits paid	(43.0)	(38.6)
Settlements	(3.6)	(11.0)
Fair value of plan assets at December 31,	482.4	392.1	372.3
Unfunded status	(124.4)	(97.1)
Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1,	98.6	82.6
Hawaiian Telcom opening balance sheet adjustment	51.2	0.0
Service cost	0.6	0.2	0.3
Interest cost	4.2	3.2	3.3
Actuarial (gain) loss	(20.3)	7.6
Benefits paid	(13.1)	(11.6)
Retiree drug subsidy received	0.3	0.2
Transfer of special death benefit	0.0	14.0
Settlements	0.0	0.0
Other	2.2	2.4
Benefit obligation at December 31,	123.7	98.6	82.6
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1,	7.5	8.7
Hawaiian Telcom opening balance sheet adjustment	0.0	0.0
Actual (loss) return on plan assets	0.3	0.2
Employer contributions	10.8	10.0
Retiree drug subsidy received	0.3	0.2
Benefits paid	(13.1)	(11.6)
Settlements	0.0	0.0
Fair value of plan assets at December 31,	5.8	7.5	$ 8.7
Unfunded status	(117.9)	(91.1)
Cincinnati Plans [Member] | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Transfer of special death benefit		(14.0)
Settlements		11.3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Settlements		$ (11.0)
Hawaii Plans [Member] | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Settlements	3.6
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Settlements	$ (3.6)