Fair Value Measurements - Narrative (Details) - GBP (£) £ in Millions	6 Months Ended
	Mar. 27, 2020	Sep. 30, 2019
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward currency contracts	£ 44.1	£ 45.0
Measurement Input, Expected Term | Minimum | Foreign Exchange Contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit derivative, term	6 months
Measurement Input, Expected Term | Maximum | Foreign Exchange Contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit derivative, term	5 years