Derivative Financial Instruments - Narrative (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Net foreign exchange contract gains (losses) reclassified from AOCI	$ 0		$ 0	$ 0
Maximum length of time being hedged for exposure to variability in future cash flows of forecasted foreign currency denominated transactions		23 months
Cash Flow Hedges
Derivative [Line Items]
Derivative instruments, gain (loss) recognized in income, ineffective portion and amount excluded from effectiveness testing, net			0	$ 0
Securities Available for Sale
Derivative [Line Items]
Estimated net gain (loss) to be reclassified into earnings within the next twelve months relating to cash flow hedges		$ (1,200,000)
Foreign Currency Denominate
Derivative [Line Items]
Estimated net gain (loss) to be reclassified into earnings within the next twelve months relating to cash flow hedges		3,100,000
Foreign Currency Denominated Investment Securities
Derivative [Line Items]
Estimated net gain (loss) to be reclassified into earnings within the next twelve months relating to cash flow hedges		1,400,000
Foreign Exchange Contracts | Net Investment Hedges
Derivative [Line Items]
Other comprehensive income (loss), unrealized gain (loss) on derivatives arising during period, before tax		$ 173,000,000	$ (223,200,000)