Allowance for Credit Losses and Credit Quality - Schedule of Allowance for Loan Losses for Covered and Non-Covered Loan Portfolios (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	$ 140,571	$ 140,891
Provision for loan and lease losses	24,067	15,581
Transfer of balance to OREO and other	(2,863)	(3,943)
Charge-offs	(19,194)	(22,734)
Recoveries	3,805	6,781
Allowance for loan and lease losses at end of period	146,386	136,576
Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	14,830	13,208
Balance created in acquisition accounting	0	900
Provision for unfunded lending commitments	451	325
Reserve for unfunded commitments at end of period	15,281	14,433
Allowance for credit losses at end of period	$ 161,667	$ 151,009