Allowance for Credit Losses and Credit Quality - Schedule of Allowance for Loan Losses for Covered and Non-Covered Loan Portfolios (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Allowance for credit losses
Allowance for loan losses at beginning of period	$ 140,571	$ 140,891
Provision for loan and lease losses	12,612	7,987
Transfer of balance to OREO and other	(2,885)	(48)
Loans charged-off	(8,918)	(9,116)
Recoveries	1,586	4,813
Allowance for loan losses at end of period	142,966	144,527
Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	14,830	13,208
Provision For Unfunded Commitments	1,151	224
Reserve for unfunded commitments at end of period	15,981	13,432
Allowance for credit losses at end of period	$ 158,947	$ 157,959