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, 2017	Sep. 30, 2016
Allowance for credit losses
Allowance for loan losses at beginning of period	$ 144,719	$ 138,378
Provision for loan losses before benefit attributable to FDIC loss share agreements		38,015
Adjustment attributable to FDIC loss share arrangements		(1,240)
Net provision for loan losses	36,718	39,255
Transfer of balance to OREO and other	963	(2,045)
Loans charged-off	(49,939)	(30,054)
Recoveries	4,167	3,899
Allowance for loan losses at end of period	136,628	148,193
Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	11,241	14,145
Balance created in purchase accounting	7,626
Provision for unfunded lending commitments	2,165	(2,155)
Reserve for unfunded commitments at end of period	21,032	11,990
Allowance for credit losses at end of period	157,660	160,183
Legacy Loans
Allowance for credit losses
Allowance for loan losses at beginning of period	105,569	93,808
Provision for loan losses before benefit attributable to FDIC loss share agreements		40,516
Adjustment attributable to FDIC loss share arrangements		0
Net provision for loan losses	37,835	40,516
Transfer of balance to OREO and other	0	0
Loans charged-off	(47,100)	(28,559)
Recoveries	3,042	3,124
Allowance for loan losses at end of period	99,346	108,889
Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	11,241	14,145
Balance created in purchase accounting	7,626
Provision for unfunded lending commitments	2,165	(2,155)
Reserve for unfunded commitments at end of period	21,032	11,990
Allowance for credit losses at end of period	120,378	120,879
Acquired Loans
Allowance for credit losses
Allowance for loan losses at beginning of period	39,150	44,570
Provision for loan losses before benefit attributable to FDIC loss share agreements		(2,501)
Adjustment attributable to FDIC loss share arrangements		(1,240)
Net provision for loan losses	(1,117)	(1,261)
Transfer of balance to OREO and other	963	(2,045)
Loans charged-off	(2,839)	(1,495)
Recoveries	1,125	775
Allowance for loan losses at end of period	37,282	39,304
Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	0	0
Balance created in purchase accounting	0
Provision for unfunded lending commitments	0	0
Reserve for unfunded commitments at end of period	0	0
Allowance for credit losses at end of period	$ 37,282	$ 39,304