Derivative Financial Instruments - Summary of Outstanding Foreign Exchange Derivatives Designated as Net Investment Hedges of Foreign Currency Risk (Detail) - Sell [Member] - Designated as Hedging Instrument [Member] - Foreign Exchange Forward [Member] - Net Investment Hedges [Member] € in Thousands, £ in Thousands, $ in Thousands	Jun. 30, 2018 EUR (€) DerivativeInstrument	Jun. 30, 2018 GBP (£) DerivativeInstrument	Jun. 30, 2018 CAD ($) DerivativeInstrument	Dec. 31, 2017 GBP (£) DerivativeInstrument	Dec. 31, 2017 CAD ($) DerivativeInstrument
EUR [Member]
Derivative [Line Items]
Number of Instruments	1	1	1
Notional Amount | €	€ 185,000
GBP [Member]
Derivative [Line Items]
Number of Instruments	2	2	2	1	1
Notional Amount | £		£ 133,100		£ 112,700
CAD [Member]
Derivative [Line Items]
Number of Instruments	1	1	1	1	1
Notional Amount | $			$ 73,100		$ 95,100