Details of Interest Income on Non accrual loans (Detail) - Non-Covered Loans - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income that would have been recorded	$ 4,145	$ 4,974	$ 3,128
Interest income actually recorded	(3,480)	(2,904)	(1,708)
Interest income foregone	$ 665	$ 2,070	$ 1,420