Shareholders' Equity - Allocated Compensation Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Apr. 28, 2018	Apr. 29, 2017	Apr. 30, 2016
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	$ 4.0	$ 12.4	$ 7.4
2014 Incentive Plan
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	4.0	12.1	6.4
2014 Incentive Plan | RSAs
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	(1.0)	6.6	3.6
2014 Incentive Plan | RSUs
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	5.0	5.5	2.8
2010 Stock Plan
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	0.0	0.2	0.4
2010 Stock Plan | RSUs
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	0.0	0.1	0.1
2010 Stock Plan | Stock Options
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	0.0	0.1	0.3
2007 Stock Plan
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	0.0	0.1	0.6
2007 Stock Plan | Stock Options
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	$ 0.0	$ 0.1	$ 0.6