Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings	Total	Treasury Stock [Member]
Balance (in shares) at Sep. 30, 2017		287,853				12,560
Balance at Sep. 30, 2017	$ 2,784.7	$ 2.9	$ 5,630.8	$ (2,382.7)	$ (16.2)	$ (450.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		231
Exercise of stock options	5.8		5.8
Vesting of restricted stock units, net of shares withheld for employee taxes ( in shares)		666
Vesting of restricted stock units, net of shares withheld for employee taxes	(14.3)		(14.3)
Stock-based compensation expense	16.4		16.4
Reacquisition of equity component from convertible notes repurchase, net of taxes	(9.8)		(9.8)
Net loss	406.7			406.7
Other comprehensive income activity	4.5				4.5
Balance (in shares) at Dec. 30, 2017		288,750				12,560
Balance at Dec. 30, 2017	3,194.0	$ 2.9	5,628.9	(1,976.0)	(11.7)	$ (450.1)
Balance (in shares) at Sep. 30, 2017		287,853				12,560
Balance at Sep. 30, 2017	2,784.7	$ 2.9	5,630.8	(2,382.7)	(16.2)	$ (450.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(274.7)
Balance (in shares) at Mar. 31, 2018		289,187				15,376
Balance at Mar. 31, 2018	2,445.7	$ 2.9	5,657.4	(2,657.4)	(0.6)	$ (556.6)
Balance (in shares) at Dec. 30, 2017		288,750				12,560
Balance at Dec. 30, 2017	3,194.0	$ 2.9	5,628.9	(1,976.0)	(11.7)	$ (450.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		154
Exercise of stock options	2.9		2.9
Vesting of restricted stock units, net of shares withheld for employee taxes ( in shares)		79
Vesting of restricted stock units, net of shares withheld for employee taxes	(1.2)		(1.2)
Common stock issued under the employee stock purchase plan (in shares)		204
Common stock issued under the employee stock purchase plan	7.4		7.4
Stock-based compensation expense	19.5		19.5
Reacquisition of equity component from convertible notes repurchase, net of taxes	(0.1)		(0.1)
Net loss	(681.4)			(681.4)
Other comprehensive income activity	11.1				11.1
Repurchase of common stock (in shares)						2,816
Repurchase of common stock	(106.5)					$ (106.5)
Balance (in shares) at Mar. 31, 2018		289,187				15,376
Balance at Mar. 31, 2018	2,445.7	$ 2.9	5,657.4	(2,657.4)	(0.6)	$ (556.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		193
Exercise of stock options	3.9		3.9
Vesting of restricted stock units, net of shares withheld for employee taxes ( in shares)		25
Vesting of restricted stock units, net of shares withheld for employee taxes	(0.5)		(0.5)
Stock-based compensation expense	17.2		17.2
Reacquisition of equity component from convertible notes repurchase, net of taxes	(30.9)		(30.9)
Net loss	112.9			112.9
Other comprehensive income activity	(18.0)				(18.0)
Repurchase of common stock (in shares)						2,161
Repurchase of common stock	(80.8)					$ (80.8)
Balance (in shares) at Jun. 30, 2018		289,405				17,537
Balance at Jun. 30, 2018	2,449.5	$ 2.9	5,647.1	(2,544.5)	(18.6)	$ (637.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		218
Exercise of stock options	4.8		4.8
Vesting of restricted stock units, net of shares withheld for employee taxes ( in shares)		33
Vesting of restricted stock units, net of shares withheld for employee taxes	(0.6)		(0.6)
Common stock issued under the employee stock purchase plan (in shares)		244
Common stock issued under the employee stock purchase plan	8.2		8.2
Stock-based compensation expense	11.9		11.9
Reacquisition of equity component from convertible notes repurchase, net of taxes	(0.1)		(0.1)
Net loss	50.5			50.5
Other comprehensive income activity	(6.9)				(6.9)
Repurchase of common stock (in shares)						2,275
Repurchase of common stock	(88.5)					$ (88.5)
Balance (in shares) at Sep. 29, 2018		289,900				19,812
Balance at Sep. 29, 2018	2,428.8	$ 2.9	5,671.3	(2,494.0)	(25.5)	$ (725.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting standard transition adjustment | Accounting Standards Update 2014-09	6.4			6.4
Accounting standard transition adjustment | Accounting Standards Update 2016-16 [Member]	(2.5)			(2.5)
Exercise of stock options (in shares)		373
Exercise of stock options	9.1		9.1
Vesting of restricted stock units, net of shares withheld for employee taxes ( in shares)		575
Vesting of restricted stock units, net of shares withheld for employee taxes	(11.6)		(11.6)
Stock-based compensation expense	17.1		17.1
Net loss	98.6
Other comprehensive income activity	(6.4)				(6.4)
Repurchase of common stock (in shares)						3,712
Repurchase of common stock	(150.1)					$ (150.1)
Balance (in shares) at Dec. 29, 2018		290,848				23,524
Balance at Dec. 29, 2018	2,394.4	$ 2.9	5,685.9	(2,386.5)	(31.9)	$ (876.0)
Balance (in shares) at Sep. 29, 2018		289,900				19,812
Balance at Sep. 29, 2018	2,428.8	$ 2.9	5,671.3	(2,494.0)	(25.5)	$ (725.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(174.0)
Balance (in shares) at Mar. 30, 2019		291,561				23,524
Balance at Mar. 30, 2019	2,159.8	$ 2.9	5,722.5	(2,659.1)	(30.5)	$ (876.0)
Balance (in shares) at Dec. 29, 2018		290,848				23,524
Balance at Dec. 29, 2018	2,394.4	$ 2.9	5,685.9	(2,386.5)	(31.9)	$ (876.0)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		454
Exercise of stock options	11.6		11.6
Vesting of restricted stock units, net of shares withheld for employee taxes ( in shares)		33
Vesting of restricted stock units, net of shares withheld for employee taxes	(0.4)		(0.4)
Common stock issued under the employee stock purchase plan (in shares)		226
Common stock issued under the employee stock purchase plan	7.9		7.9
Stock-based compensation expense	17.5		17.5
Net loss	(272.6)
Other comprehensive income activity	1.4				1.4
Balance (in shares) at Mar. 30, 2019		291,561				23,524
Balance at Mar. 30, 2019	$ 2,159.8	$ 2.9	$ 5,722.5	$ (2,659.1)	$ (30.5)	$ (876.0)