Description of business and summary of significant accounting policies - Earning Per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Oct. 02, 2016	Jul. 03, 2016	Apr. 03, 2016	Jan. 03, 2016	Sep. 27, 2015	Jun. 28, 2015	Mar. 29, 2015	Dec. 28, 2014	Oct. 02, 2016	Sep. 27, 2015	Sep. 28, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Document Fiscal Year Focus									2016
Stock awards excluded from computation of diluted EPS										0.2	2.8
BASIC
Net income (loss)	$ 40.5	$ 115.2	$ (212.0)	$ 23.7	$ 25.3	$ 14.7	$ 24.9	$ 19.7	$ (32.6)	$ 84.6	$ 23.1
Weighted-average common shares outstanding									107.0	94.2	92.9
Basic earnings (loss) per share	$ 0.36	$ 1.03	$ (1.93)	$ 0.25	$ 0.27	$ 0.16	$ 0.26	$ 0.21	$ (0.31)	$ 0.90	$ 0.25
DILUTED
Net income (loss)	$ 40.5	$ 115.2	$ (212.0)	$ 23.7	$ 25.3	$ 14.7	$ 24.9	$ 19.7	$ (32.6)	$ 84.6	$ 23.1
Weighted-average common shares outstanding									107.0	94.2	92.9
Dilutive effect of stock awards									0.0	1.7	1.6
Weighted-average common shares outstanding on a diluted basis									107.0	95.9	94.5
Diluted earnings (loss) per share	$ 0.35	$ 1.00	$ (1.93)	$ 0.25	$ 0.26	$ 0.15	$ 0.26	$ 0.21	$ (0.31)	$ 0.88	$ 0.24