Balance Sheet Components - Available-for-Sale Securities (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 29, 2017	Sep. 29, 2017	Sep. 30, 2016	Oct. 02, 2015
Schedule of Available-for-sale Securities [Line Items]
Impairment charges		$ 51,400,000	$ 0	$ 0
Available-for-sale Securities:
Amortized Cost	$ 59,800,000	59,800,000	95,300,000
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Fair Value	59,800,000	59,800,000	95,300,000
Original CPTC Loans
Available-for-sale Securities:
Amortized Cost	47,400,000	47,400,000	95,300,000
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Fair Value	47,400,000	47,400,000	95,300,000
DRTC Loans
Available-for-sale Securities:
Amortized Cost	8,000,000	8,000,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	8,000,000	8,000,000
GPTC securities
Available-for-sale Securities:
Amortized Cost	4,400,000	4,400,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	4,400,000	4,400,000
Original CPTC loans (3)
Schedule of Available-for-sale Securities [Line Items]
Impairment charges	13,100,000	51,400,000
Available-for-sale Securities:
Fair Value	$ 47,400,000	$ 47,400,000	$ 95,300,000