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Balance Sheet Components - Available-for-Sale Securities (Detail) - USD ($)	12 Months Ended
	Sep. 29, 2017	Sep. 30, 2016	Sep. 28, 2018
Debt Securities, Available-for-sale [Line Items]
Cash equivalents			$ 44,100,000
Amortized Cost	$ 60,100,000		62,500,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Fair Value	60,100,000		62,500,000
Impairment charges	51,400,000	$ 0
MPTC Series B-1 Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost			25,100,000
Gross Unrealized Gains			0
Gross Unrealized Losses			0
Fair Value			25,100,000
MPTC Series B-2 Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost			23,100,000
Gross Unrealized Gains			0
Gross Unrealized Losses			0
Fair Value			23,100,000
GPTC securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,400,000		7,900,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Fair Value	4,400,000		7,900,000
APTC Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost			6,400,000
Gross Unrealized Gains			0
Gross Unrealized Losses			0
Fair Value			6,400,000
Original CPTC Loans
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	47,400,000
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	47,400,000
DRTC Loans
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,300,000
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	8,300,000
Original CPTC loans
Debt Securities, Available-for-sale [Line Items]
Impairment charges	$ 51,400,000
Money Market Funds
Debt Securities, Available-for-sale [Line Items]
Cash equivalents			$ 44,100,000