Financial Derivatives and Hedging Activities - Outstanding Derivatives Used to Hedge Foreign Exchange Risks (Detail)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 EUR (€)	Sep. 30, 2018 PHP (₱)	Sep. 30, 2018 CAD ($)	Sep. 30, 2018 GBP (£)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 PHP (₱)	Dec. 31, 2017 CAD ($)	Dec. 31, 2017 GBP (£)
Cash Flow Hedges [Member] | Philippine Peso / British Pound [Member] | Designated as Hedging [Member] | Sell Notional [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative notional amount | ₱								₱ 19,047,000
Cash Flow Hedges [Member] | Euro / British Pound [Member] | Designated as Hedging [Member] | Sell Notional [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative notional amount | €		€ 16,130,000					€ 13,586,000
Cash Flow Hedges [Member] | Euro / U.S. Dollar [Member] | Designated as Hedging [Member] | Sell Notional [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative notional amount | €							1,048,000
Cash Flow Hedges [Member] | Euro / U.S. Dollar [Member] | Designated as Hedging [Member] | Buy Notional [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative notional amount | $	$ 74,569,000					$ 60,519,000
Cash Flow Hedges [Member] | Euro / Philippine Peso [Member] | Designated as Hedging [Member] | Buy Notional [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative notional amount | ₱			₱ 1,147,101,000					890,096,000
Cash Flow Hedges [Member] | British Pound / Philippine Peso [Member] | Designated as Hedging [Member] | Buy Notional [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative notional amount | ₱			₱ 1,026,551,000					₱ 797,496,000
Cash Flow Hedges [Member] | Canadian Dollar / U.S. Dollar | Designated as Hedging [Member] | Sell Notional [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative notional amount | $	168,000
Cash Flow Hedges [Member] | U.S. Dollar / Canadian Dollar [Member] | Designated as Hedging [Member] | Buy Notional [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative notional amount | $				$ 31,394,000					$ 32,265,000
Cash Flow Hedges [Member] | U.S Dollar / Euro [Member] | Designated as Hedging [Member] | Sell Notional [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative notional amount | €		414,000					946,000
Cash Flow Hedges [Member] | U.S Dollar / Euro [Member] | Designated as Hedging [Member] | Buy Notional [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative notional amount | €							4,253,000
Cash Flow Hedges [Member] | British Pound / Euro [Member] | Designated as Hedging [Member] | Buy Notional [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative notional amount | €							335,000
Foreign Currency Hedges [Member] | Euro / U.S. Dollar [Member] | Not Designated as Hedging [Member] | Buy Notional [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative notional amount | $	11,000,000					4,500,000
Foreign Currency Hedges [Member] | Canadian Dollar / U.S. Dollar | Not Designated as Hedging [Member] | Sell Notional [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative notional amount | $				$ 2,500,000
Foreign Currency Hedges [Member] | British Pound / Euro [Member] | Not Designated as Hedging [Member] | Sell Notional [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative notional amount | £					£ 1,000,000					£ 1,000,000
Foreign Currency Hedges [Member] | British Pound / Euro [Member] | Not Designated as Hedging [Member] | Buy Notional [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative notional amount | €		€ 10,000,000					€ 13,000,000
Foreign Currency Hedges [Member] | U.S. Dollar / British Pound [Member] | Not Designated as Hedging [Member] | Sell Notional [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative notional amount | $	$ 22,500,000					$ 17,500,000