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, 2017	Jun. 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		25,974
Adjustment attributable to FDIC loss share arrangements		(797)
Net provision for loan losses	18,204	26,771
Transfer of balance to OREO and other	258	(967)
Loans charged-off	(19,480)	(18,555)
Recoveries	2,524	2,622
Allowance for loan losses at end of period	146,225	147,452
Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	11,241	14,145
Provision for (Reversal of) unfunded lending commitments	(779)	(319)
Reserve for unfunded commitments at end of period	10,462	13,826
Allowance for credit losses at end of period	156,687	161,278
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		28,390
Adjustment attributable to FDIC loss share arrangements		0
Net provision for loan losses	18,315	28,390
Transfer of balance to OREO and other	0	0
Loans charged-off	(18,098)	(17,359)
Recoveries	1,824	2,022
Allowance for loan losses at end of period	107,610	106,861
Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	11,241	14,145
Provision for (Reversal of) unfunded lending commitments	(779)	(319)
Reserve for unfunded commitments at end of period	10,462	13,826
Allowance for credit losses at end of period	118,072	120,687
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		(2,416)
Adjustment attributable to FDIC loss share arrangements		(797)
Net provision for loan losses	(111)	(1,619)
Transfer of balance to OREO and other	258	(967)
Loans charged-off	(1,382)	(1,196)
Recoveries	700	600
Allowance for loan losses at end of period	38,615	40,591
Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	0	0
Provision for (Reversal of) unfunded lending commitments	0	0
Reserve for unfunded commitments at end of period	0	0
Allowance for credit losses at end of period	$ 38,615	$ 40,591