Loans and Lease Finance Receivables and Allowance for Loan Losses - Summary of Components of Loans and Lease Finance Receivables, Excluding PCI Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans, excluding PCI loans, net of deferred loan fees	$ 4,746,424		$ 4,395,064
Less: Allowance for loan losses	(60,631)	$ (60,201)	(61,540)	$ (61,001)	$ (60,938)	$ (59,156)
Total loans and lease finance receivables	4,685,793		4,333,524
PCI Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	1,002		2,309
PCI Loans	37,306		73,093
SBA	1,410		327
Discount on PCI loans	(758)		(1,508)
Commercial real estate	33,799		67,594
Construction	0		0
SFR mortgage	166		178
Dairy & livestock and agribusiness	335		1,216
Municipal lease finance receivables	0		0
Consumer and other loans	594		1,469
Less: Allowance for loan losses	(431)	$ (659)	(1,219)	$ (600)	$ (310)	$ 0
Net loans	36,117		70,366
Loans, Excluding PCI Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	528,659		485,078
SBA	124,091		97,184
Commercial real estate	3,332,517		2,930,141
Construction	74,148		85,879
SFR mortgage	244,662		250,605
Dairy & livestock and agribusiness	270,482		338,631
Municipal lease finance receivables	71,352		64,639
Consumer and other loans	70,415		78,274
Total gross loans, excluding PCI loans	4,716,326		4,330,431
Less: Deferred loan fees, net	(6,450)		(6,952)
Gross loans, excluding PCI loans, net of deferred loan fees	4,709,876		4,323,479
Less: Allowance for loan losses	(60,200)		(60,321)
Net loans	$ 4,649,676		$ 4,263,158