Fair Value Measurements - Assets and Liabilities Measures On a Gross Basis (Details) - Fair Value, Measurements, Recurring $ in Thousands, £ in Millions	Dec. 28, 2018 USD ($)	Dec. 28, 2018 GBP (£)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 GBP (£)
Assets
Forward currency contracts | £		£ 46.4		£ 49.1
Level 1
Assets
Cash equivalents	$ 38,771		$ 28,175
Forward currency contracts	0		0
Liabilities
Forward currency contracts	0		0
Level 2
Assets
Cash equivalents	0		0
Forward currency contracts	727		0
Liabilities
Forward currency contracts	68		1,857
Level 3
Assets
Cash equivalents	0		0
Forward currency contracts	0		0
Liabilities
Forward currency contracts	$ 0		$ 0