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		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loans losses at beginning of period			$ 130,131	$ 143,074
Provision for loan losses before benefit attributable to FDIC loss share agreements			17,855	13,718
Adjustment attributable to FDIC loss share arrangements			(1,342)	1,153
Provision for loan losses	$ 5,062	$ 5,714	19,197	12,565
Transfer of balance to OREO			(566)	(5,378)
Transfer of balance to non-covered			0
Loans charged-off			(22,234)	(21,651)
Recoveries			5,068	4,777
Allowance for loans losses at end of period	130,254	134,540	130,254	134,540
Financing Receivable, Allowance For Credit Losses, Unfunded Commitments [Roll Forward]
Reserve for unfunded commitments at beginning of period			11,801	11,147
Provision for unfunded lending commitments			2,724	952
Reserve for unfunded commitments at end of period	14,525	12,099	14,525	12,099
Allowance for credit losses at end of period	144,779	146,639	144,779	146,639
Non-Covered Legacy Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loans losses at beginning of period			76,174	67,342
Provision for loan losses before benefit attributable to FDIC loss share agreements			17,743	9,635
Adjustment attributable to FDIC loss share arrangements			0	0
Provision for loan losses			17,743	9,635
Transfer of balance to OREO			0	0
Transfer of balance to non-covered			0
Loans charged-off			(12,073)	(8,242)
Recoveries			4,556	4,338
Allowance for loans losses at end of period	86,400	73,073	86,400	73,073
Financing Receivable, Allowance For Credit Losses, Unfunded Commitments [Roll Forward]
Reserve for unfunded commitments at beginning of period			11,801	11,147
Provision for unfunded lending commitments			2,724	952
Reserve for unfunded commitments at end of period	14,525	12,099	14,525	12,099
Allowance for credit losses at end of period	100,925	85,172	100,925	85,172
Non-Covered Acquired Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loans losses at beginning of period			9,193	4,557
Provision for loan losses before benefit attributable to FDIC loss share agreements			143	(1,795)
Adjustment attributable to FDIC loss share arrangements			0	0
Provision for loan losses			143	(1,795)
Transfer of balance to OREO			(209)	(207)
Transfer of balance to non-covered			28,700
Loans charged-off			(8,973)	(586)
Recoveries			504	401
Allowance for loans losses at end of period	29,358	2,370	29,358	2,370
Financing Receivable, Allowance For Credit Losses, Unfunded Commitments [Roll Forward]
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	0	0
Allowance for credit losses at end of period	29,358	2,370	29,358	2,370
Covered Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loans losses at beginning of period			44,764	71,175
Provision for loan losses before benefit attributable to FDIC loss share agreements			(31)	5,878
Adjustment attributable to FDIC loss share arrangements			(1,342)	1,153
Provision for loan losses			1,311	4,725
Transfer of balance to OREO			(357)	(5,171)
Transfer of balance to non-covered			(28,700)
Loans charged-off			(1,188)	(12,823)
Recoveries			8	38
Allowance for loans losses at end of period	14,496	59,097	14,496	59,097
Financing Receivable, Allowance For Credit Losses, Unfunded Commitments [Roll Forward]
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	0	0
Allowance for credit losses at end of period	$ 14,496	$ 59,097	$ 14,496	$ 59,097