Derivative Financial Instruments - Summary of Outstanding Forward Foreign Currency Contracts by Currency (Detail) - Foreign Currency Forward Contracts - Not Designated as Hedging Instrument € in Thousands, £ in Thousands, $ in Thousands, $ in Thousands	Sep. 30, 2018 GBP (£)	Sep. 30, 2018 SGD ($)	Sep. 30, 2018 EUR (€)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 GBP (£)	Sep. 30, 2017 EUR (€)	Sep. 30, 2017 USD ($)
Foreign Exchange Contracts To Sell European Euro for US Dollar | Short
Derivative [Line Items]
Contract amount of forward foreign currency contracts			€ 9,000	$ 10,372		€ 5,050	$ 5,968
Fair value of forward foreign currency contracts to sell and buy foreign currency				0			0
Foreign Exchange Contracts To Purchase British Pounds With US Dollars | Long
Derivative [Line Items]
Contract amount of forward foreign currency contracts	£ 8,598			11,200	£ 9,341		12,500
Fair value of forward foreign currency contracts to sell and buy foreign currency				0			$ 0
Foreign Exchange Contracts To Purchase Singapore Dollars With United States Dollars | Long
Derivative [Line Items]
Contract amount of forward foreign currency contracts		$ 9,580		7,000
Fair value of forward foreign currency contracts to sell and buy foreign currency				$ 0