Allowance for Credit Losses and Credit Quality - Schedule of Allowance for Loan Losses for Covered and Non-Covered Loan Portfolios (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 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	32,190	26,678
Transfer of balance to OREO and other	(2,696)	(5,709)
Charge-offs	(29,971)	(34,740)
Recoveries	6,141	9,830
Allowance for loan and lease losses at end of period	146,235	136,950
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	1,314	613
Reserve for unfunded commitments at end of period	16,144	14,721
Allowance for credit losses at end of period	$ 162,379	$ 151,671