Activity in Allowance for Losses on Covered Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation Allowance [Line Items]
Balance, beginning of period	$ 23,701	$ 31,395
Recovery of losses on covered loans	37,242	4,180	$ (15,004)
Balance, end of period	0	23,701	$ 31,395
Covered | Covered loans held for sale
Valuation Allowance [Line Items]
Recovery of losses on covered loans	$ (23,701)	$ (7,694)