Fair Value Measurements - Assets and Liabilities Measures On a Gross Basis (Details) - Fair Value, Measurements, Recurring $ in Thousands, £ in Millions	Jun. 26, 2020 GBP (£)	Jun. 26, 2020 USD ($)	Sep. 30, 2019 GBP (£)	Sep. 30, 2019 USD ($)
Assets
Forward currency contracts | £	£ 44.1		£ 45.0
Level 1
Assets
Cash equivalents		$ 175,166		$ 72,132
Forward currency contracts		0		0
Liabilities
Forward currency contracts		0		0
Level 2
Assets
Cash equivalents		0		0
Forward currency contracts		4,669		3,420
Liabilities
Forward currency contracts		144		0
Level 3
Assets
Cash equivalents		0		0
Forward currency contracts		0		0
Liabilities
Forward currency contracts		$ 0		$ 0