Description of business and summary of significant accounting policies - Earning Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Oct. 01, 2017	Jul. 02, 2017	Apr. 02, 2017	Jan. 01, 2017	Oct. 02, 2016	Jul. 03, 2016	Apr. 03, 2016	Jan. 03, 2016	Oct. 01, 2017	Oct. 02, 2016	Sep. 27, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Stock awards excluded from computation of diluted EPS									0		200,000
BASIC
Net income (loss)	$ 102.0	$ 13.6	$ 41.2	$ 19.5	$ 40.5	$ 115.2	$ (212.0)	$ 23.7	$ 176.3	$ (32.6)	$ 84.6
Weighted-average common shares outstanding									114,900,000	107,000,000	94,200,000
Basic earnings (loss) per share	$ 0.88	$ 0.12	$ 0.36	$ 0.17	$ 0.36	$ 1.03	$ (1.93)	$ 0.25	$ 1.54	$ (0.31)	$ 0.90
DILUTED
Net income (loss)	$ 102.0	$ 13.6	$ 41.2	$ 19.5	$ 40.5	$ 115.2	$ (212.0)	$ 23.7	$ 176.3	$ (32.6)	$ 84.6
Weighted-average common shares outstanding									114,900,000	107,000,000	94,200,000
Dilutive effect of stock awards									2,200,000	0	1,700,000
Weighted-average common shares outstanding on a diluted basis									117,100,000	107,000,000	95,900,000
Diluted earnings (loss) per share	$ 0.87	$ 0.12	$ 0.35	$ 0.17	$ 0.35	$ 1.00	$ (1.93)	$ 0.25	$ 1.51	$ (0.31)	$ 0.88