Description of business and summary of significant accounting policies - Fair Value Measurements Using (Detail) - Measured on a recurring basis - USD ($) $ in Millions	Oct. 01, 2017	Oct. 02, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable Securities, Current	$ 1.3	$ 1.3
Fair Value, Inputs, Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable Securities, Current	1.3	1.3
Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable Securities, Current	0.0	0.0
Fair Value, Inputs, Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable Securities, Current	$ 0.0	$ 0.0