Fair Value - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 29, 2017	Sep. 30, 2016
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Available-for-sale securities:	$ 59.8	$ 95.3
DRTC securities
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Available-for-sale securities:	8.0
GPTC securities
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Available-for-sale securities:	4.4
Fair Value, Measurements, Recurring
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	59.8	95.3
Contingent consideration		(1.3)
Total liabilities measured at fair value		(1.3)
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Instruments (Level 1)
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	0.0	0.0
Contingent consideration		0.0
Total liabilities measured at fair value		0.0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	12.4	0.0
Contingent consideration		0.0
Total liabilities measured at fair value		0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Total assets measured at fair value	47.4	95.3
Contingent consideration		(1.3)
Total liabilities measured at fair value		(1.3)
Fair Value, Measurements, Recurring | Original CPTC Loans
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Available-for-sale securities:	47.4	95.3
Fair Value, Measurements, Recurring | Original CPTC Loans | Quoted Prices in Active Markets for Identical Instruments (Level 1)
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Available-for-sale securities:	0.0	0.0
Fair Value, Measurements, Recurring | Original CPTC Loans | Significant Other Observable Inputs (Level 2)
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Available-for-sale securities:	0.0	0.0
Fair Value, Measurements, Recurring | Original CPTC Loans | Significant Unobservable Inputs (Level 3)
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Available-for-sale securities:	47.4	$ 95.3
Fair Value, Measurements, Recurring | DRTC securities
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Available-for-sale securities:	8.0
Fair Value, Measurements, Recurring | DRTC securities | Quoted Prices in Active Markets for Identical Instruments (Level 1)
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Available-for-sale securities:	0.0
Fair Value, Measurements, Recurring | DRTC securities | Significant Other Observable Inputs (Level 2)
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Available-for-sale securities:	8.0
Fair Value, Measurements, Recurring | DRTC securities | Significant Unobservable Inputs (Level 3)
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Available-for-sale securities:	0.0
Fair Value, Measurements, Recurring | GPTC securities
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Available-for-sale securities:	4.4
Fair Value, Measurements, Recurring | GPTC securities | Quoted Prices in Active Markets for Identical Instruments (Level 1)
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Available-for-sale securities:	0.0
Fair Value, Measurements, Recurring | GPTC securities | Significant Other Observable Inputs (Level 2)
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Available-for-sale securities:	4.4
Fair Value, Measurements, Recurring | GPTC securities | Significant Unobservable Inputs (Level 3)
Assets/Liabilities Measured at Fair Value on a Recurring Basis
Available-for-sale securities:	$ 0.0