Derivative Financial Instruments - Summary of Outstanding Forward Foreign Currency Contracts by Currency (Detail) € in Thousands, £ in Thousands, $ in Thousands	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 GBP (£)	Sep. 30, 2017 EUR (€)	Sep. 30, 2017 USD ($)
Foreign Exchange Contracts To Sell European Euro for US Dollar
Derivative [Line Items]
Fair value of forward foreign currency contracts to sell and buy foreign currency			$ 0			$ 0
Foreign Exchange Contracts To Sell European Euro for US Dollar | Foreign currency forward contracts | Short | Not Designated as Hedging Instrument
Derivative [Line Items]
Contract amount of forward foreign currency contracts		€ 7,350	8,825		€ 5,050	5,968
Foreign Exchange Contracts To Purchase British Pounds With US Dollars
Derivative [Line Items]
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 | Foreign currency forward contracts | Short | Not Designated as Hedging Instrument
Derivative [Line Items]
Contract amount of forward foreign currency contracts	£ 5,398		7,300	£ 9,341		$ 12,500
Foreign Exchange Contracts To Purchase Singapore Dollars With US Dollars
Derivative [Line Items]
Fair value of forward foreign currency contracts to sell and buy foreign currency			0
Foreign Exchange Contracts To Purchase Singapore Dollars With US Dollars | Foreign currency forward contracts | Short | Not Designated as Hedging Instrument
Derivative [Line Items]
Contract amount of forward foreign currency contracts	£ 7,734		$ 5,800