Description of business and summary of significant accounting policies - Fair Value Measurements Using (Detail) - USD ($) $ in Millions	Sep. 27, 2015	Sep. 28, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swap liabilities		$ 0.1
Fair Value, Inputs, Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swap liabilities		0.0
Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swap liabilities		0.1
Fair Value, Inputs, Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swap liabilities		0.0
Measured on a recurring basis
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable Securities, Current	$ 1.7	4.1
Measured on a recurring basis | Fair Value, Inputs, Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable Securities, Current	1.7	4.1
Measured on a recurring basis | Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable Securities, Current	0.0	0.0
Measured on a recurring basis | Fair Value, Inputs, Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable Securities, Current	$ 0.0	$ 0.0