Fair Value - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 28, 2018	Sep. 29, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Cash equivalents	$ 44.1
Available-for- sale securities	62.5	$ 60.1
MPTC Series B-2 Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	23.1
APTC Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	6.4
GPTC securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	7.9	4.4
DRTC securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities		8.3
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Total assets measured at fair value	106.6	60.1
Contingent consideration	(24.4)
Total liabilities measured at fair value	(24.4)
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Instruments (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Total assets measured at fair value	44.1	0.0
Contingent consideration	0.0
Total liabilities measured at fair value	0.0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Total assets measured at fair value	62.5	12.7
Contingent consideration	0.0
Total liabilities measured at fair value	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Total assets measured at fair value	0.0	47.4
Contingent consideration	(24.4)
Total liabilities measured at fair value	(24.4)
Fair Value, Measurements, Recurring | MPTC Series B-1 Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	25.1
Fair Value, Measurements, Recurring | MPTC Series B-1 Bonds | Quoted Prices in Active Markets for Identical Instruments (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	0.0
Fair Value, Measurements, Recurring | MPTC Series B-1 Bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	25.1
Fair Value, Measurements, Recurring | MPTC Series B-1 Bonds | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	0.0
Fair Value, Measurements, Recurring | MPTC Series B-2 Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	23.1
Fair Value, Measurements, Recurring | MPTC Series B-2 Bonds | Quoted Prices in Active Markets for Identical Instruments (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	0.0
Fair Value, Measurements, Recurring | MPTC Series B-2 Bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	23.1
Fair Value, Measurements, Recurring | MPTC Series B-2 Bonds | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	0.0
Fair Value, Measurements, Recurring | APTC Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	6.4
Fair Value, Measurements, Recurring | APTC Securities | Quoted Prices in Active Markets for Identical Instruments (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	0.0
Fair Value, Measurements, Recurring | APTC Securities | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	6.4
Fair Value, Measurements, Recurring | APTC Securities | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	0.0
Fair Value, Measurements, Recurring | GPTC securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	7.9	4.4
Fair Value, Measurements, Recurring | GPTC securities | Quoted Prices in Active Markets for Identical Instruments (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	0.0	0.0
Fair Value, Measurements, Recurring | GPTC securities | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	7.9	4.4
Fair Value, Measurements, Recurring | GPTC securities | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Original CPTC Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities		47.4
Fair Value, Measurements, Recurring | Original CPTC Loans | Quoted Prices in Active Markets for Identical Instruments (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities		0.0
Fair Value, Measurements, Recurring | Original CPTC Loans | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities		0.0
Fair Value, Measurements, Recurring | Original CPTC Loans | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities		47.4
Fair Value, Measurements, Recurring | DRTC securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities		8.3
Fair Value, Measurements, Recurring | DRTC securities | Quoted Prices in Active Markets for Identical Instruments (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities		0.0
Fair Value, Measurements, Recurring | DRTC securities | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities		8.3
Fair Value, Measurements, Recurring | DRTC securities | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Available-for- sale securities		$ 0.0
Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Cash equivalents	44.1
Money Market Funds | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Cash equivalents	44.1
Money Market Funds | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Instruments (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Cash equivalents	44.1
Money Market Funds | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Cash equivalents	0.0
Money Market Funds | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Cash equivalents	$ 0.0