Fair Value Measurements - Assets and Liabilities Measures On a Gross Basis (Details) - Fair Value, Measurements, Recurring $ in Thousands, £ in Millions	Dec. 27, 2019 USD ($)	Dec. 27, 2019 GBP (£)	Sep. 30, 2019 USD ($)	Sep. 30, 2019 GBP (£)
Assets
Forward currency contracts | £		£ 45.0		£ 45.0
Level 1
Assets
Cash equivalents	$ 111,422		$ 72,132
Forward currency contracts	0		0
Level 2
Assets
Cash equivalents	0		0
Forward currency contracts	297		3,420
Level 3
Assets
Cash equivalents	0		0
Forward currency contracts	$ 0		$ 0