Retirement Plans - Defined Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Expected Amortization, Next Fiscal Year [Abstract]
Prior service cost included in accumulated other comprehensive income and expected to be recognized in net periodic pension costs	$ 0.3
Prior service cost included in Accumulated other comprehensive income and expected to be recognized in net periodic pension costs, net of tax	0.2
Actuarial loss included in accumulated other comprehensive income and expected to be recognized in net periodic pension costs	4.3
Actuarial loss included in accumulated other comprehensive income and expected to be recognized in net periodic pension costs, net of tax	3.4
Defined Benefit Plans
Amounts included in accumulated other comprehensive loss
Unrecognized actuarial losses (gross)	48.8	$ 40.2
Unrecognized actuarial losses (net of tax)	38.0	25.3
Unrecognized prior service cost (gross)	1.3	0.9
Unrecognized prior service cost (net of tax)	1.1	0.5
Projected benefit obligation
Beginning of year	172.5	174.3
Service cost	2.6	3.4	$ 3.7
Interest cost	5.3	5.4	7.1
Plan amendments	0.7	(0.1)
Actuarial (gain)/loss	15.5	1.2
Benefits paid	(13.8)	(11.7)
End of year	182.8	172.5	174.3
Fair value of plan assets
Beginning of year	163.2	162.7
Actual return on plan assets	14.9	11.2
Company contributions	0.0	0.0
Benefits paid	(13.8)	(11.7)
End of year	165.7	163.2	$ 162.7
(Unfunded) funded status end of year	(17.1)	(9.3)
Accumulated benefit obligation at end of year	181.1	171.5
Executive supplemental and director defined benefit pension plans
Projected benefit obligation
Beginning of year	21.7
End of year	22.3	21.7
Fair value of plan assets
Beginning of year	0.0
Company contributions	1.4	1.0
End of year	0.0	0.0
Accumulated benefit obligation at end of year	$ 20.7	$ 20.6