Summary of Notional Amounts of Outstanding Foreign Exchange Forward Contracts, Designated as Net Investment Hedges (Detail) - Net Investment Hedging [Member] - Contracts to Sell GBP for Euros [Member] € in Millions, £ in Millions	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 GBP (£)	Dec. 31, 2016 EUR (€)	Dec. 31, 2016 GBP (£)
Sell [Member]
Derivative [Line Items]
Derivative Notional Amount | £				£ 22.1
Buy [Member]
Derivative [Line Items]
Derivative Notional Amount | €			€ 26.4