Retirement Benefit Plans (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, total costs	$ 4.5	$ 4.0	$ 3.8
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss included in accumulated other comprehensive income (loss) expected to be recognized in net periodic benefit cost, before tax	0.5
Actuarial loss included in accumulated other comprehensive income (loss) expected to be recognized in net periodic benefit cost, net of tax	0.4
Prior service cost (credit) included in accumulated other comprehensive income (loss) expected to be recognized in net periodic benefit cost, (less than)	0.1
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss included in accumulated other comprehensive income (loss) expected to be recognized in net periodic benefit cost, before tax	0.1
Actuarial loss included in accumulated other comprehensive income (loss) expected to be recognized in net periodic benefit cost, net of tax	0.1
Prior service cost (credit) included in accumulated other comprehensive income (loss) expected to be recognized in net periodic benefit cost, (less than)	$ (0.1)