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, 2017	Mar. 31, 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		14,647
Adjustment attributable to FDIC loss share arrangements		(258)
Net provision for loan losses	6,154	14,905
Transfer of balance to OREO and other	73	(2,459)
Loans charged off	(7,291)	(5,560)
Recoveries	1,235	1,551
Allowance for loan losses at end of period	144,890	146,557
Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	11,241	14,145
Provision for unfunded lending commitments	419	(112)
Reserve for unfunded commitments at end of period	11,660	14,033
Allowance for credit losses at end of period	156,550	160,590
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		15,908
Adjustment attributable to FDIC loss share arrangements		0
Net provision for loan losses	6,566	15,908
Transfer of balance to OREO and other	0	0
Loans charged off	(7,202)	(5,389)
Recoveries	880	1,247
Allowance for loan losses at end of period	105,813	105,574
Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	11,241	14,145
Provision for unfunded lending commitments	419	(112)
Reserve for unfunded commitments at end of period	11,660	14,033
Allowance for credit losses at end of period	117,473	119,607
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		(1,261)
Adjustment attributable to FDIC loss share arrangements		(258)
Net provision for loan losses	(412)	(1,003)
Transfer of balance to OREO and other	73	(2,459)
Loans charged off	(89)	(171)
Recoveries	355	304
Allowance for loan losses at end of period	39,077	40,983
Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	0	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	$ 39,077	$ 40,983