Commitments and Contingencies - Gains (Losses) Related to Interest Rate Derivatives Designated as Hedges of Fair Value (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gain Loss On Fair Value Hedges Recognized In Earnings [Line Items]
Gains (losses) included in the consolidated statements of income	$ (744)	$ 690	$ 158
Fair value hedge | Interest rate swaps | Interest income
Gain Loss On Fair Value Hedges Recognized In Earnings [Line Items]
Gains (losses) included in the consolidated statements of income	348	320	176
Fair value hedge | Interest rate swaps | Interest expense
Gain Loss On Fair Value Hedges Recognized In Earnings [Line Items]
Gains (losses) included in the consolidated statements of income	0	6,670	1,400
Fair value hedge | Interest rate caps and swaps | Derivatives designated as hedging instruments | Interest income
Gain Loss On Fair Value Hedges Recognized In Earnings [Line Items]
Gains (losses) included in the consolidated statements of income	(348)	(320)	(176)
Fair value hedge | Interest rate caps and swaps | Derivatives designated as hedging instruments | Interest expense
Gain Loss On Fair Value Hedges Recognized In Earnings [Line Items]
Gains (losses) included in the consolidated statements of income	$ 0	$ (6,645)	$ (1,473)