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
	Sep. 27, 2015	Jun. 28, 2015	Mar. 29, 2015	Dec. 28, 2014	Sep. 28, 2014	Jun. 29, 2014	Mar. 30, 2014	Dec. 29, 2013	Sep. 27, 2015	Sep. 28, 2014	Sep. 29, 2013
BASIC
Net income (loss)	$ 25.3	$ 14.7	$ 24.9	$ 19.7	$ 32.8	$ (4.3)	$ (6.8)	$ 1.4	$ 84.6	$ 23.1	$ 43.7
Weighted-average common shares outstanding									94.2	92.9	89.5
Basic earnings (loss) per share	$ 0.27	$ 0.16	$ 0.26	$ 0.21	$ 0.35	$ (0.05)	$ (0.07)	$ 0.02	$ 0.90	$ 0.25	$ 0.49
DILUTED
Net income (loss)	$ 25.3	$ 14.7	$ 24.9	$ 19.7	$ 32.8	$ (4.3)	$ (6.8)	$ 1.4	$ 84.6	$ 23.1	$ 43.7
Weighted-average common shares outstanding									94.2	92.9	89.5
Dilutive effect of stock awards									1.7	1.6	1.8
Weighted-average common shares outstanding on a diluted basis									95.9	94.5	91.3
Diluted earnings (loss) per share	$ 0.26	$ 0.15	$ 0.26	$ 0.21	$ 0.34	$ (0.05)	$ (0.07)	$ 0.02	$ 0.88	$ 0.24	$ 0.48