Allowance for Credit Losses and Credit Quality - Schedule of Allowance for Loan Losses for Covered Loan and Non-Covered Loan Portfolios (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	$ 138,378	$ 130,131
Provision for (Reversal of) loan losses before benefit attributable to FDIC loss share agreements	14,647	3,493
Adjustment attributable to FDIC loss share arrangements	258	1,852
Net provision for loan losses	14,905	5,345
Adjustment attributable to FDIC loss share arrangements	(258)	(1,852)
Transfer of balance to OREO	(109)	(26)
Loans charged-off	(7,910)	(6,528)
Recoveries	1,551	1,243
Allowance for loan losses at end of period	146,557	128,313
Financing Receivable, Allowance For Credit Losses, Unfunded Commitments [Roll Forward]
Reserve for unfunded commitments at beginning of period	14,145	11,801
Provision for (Reversal of) unfunded lending commitments	(112)	1,048
Reserve for unfunded commitments at end of period	14,033	12,849
Allowance for credit losses at end of period	160,590	141,162
Legacy Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	93,808	76,174
Provision for (Reversal of) loan losses before benefit attributable to FDIC loss share agreements	15,908	4,177
Adjustment attributable to FDIC loss share arrangements	0	0
Net provision for loan losses	15,908	4,177
Adjustment attributable to FDIC loss share arrangements	0	0
Transfer of balance to OREO	0	0
Loans charged-off	(5,389)	(2,669)
Recoveries	1,247	1,091
Allowance for loan losses at end of period	105,574	78,773
Financing Receivable, Allowance For Credit Losses, Unfunded Commitments [Roll Forward]
Reserve for unfunded commitments at beginning of period	14,145	11,801
Provision for (Reversal of) unfunded lending commitments	(112)	1,048
Reserve for unfunded commitments at end of period	14,033	12,849
Allowance for credit losses at end of period	119,607	91,622
Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses at beginning of period	44,570	53,957
Provision for (Reversal of) loan losses before benefit attributable to FDIC loss share agreements	(1,261)	(684)
Adjustment attributable to FDIC loss share arrangements	258	1,852
Net provision for loan losses	(1,003)	1,168
Adjustment attributable to FDIC loss share arrangements	(258)	(1,852)
Transfer of balance to OREO	(109)	(26)
Loans charged-off	(2,521)	(3,859)
Recoveries	304	152
Allowance for loan losses at end of period	40,983	49,540
Financing Receivable, Allowance For Credit Losses, Unfunded Commitments [Roll Forward]
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	$ 40,983	$ 49,540