Loans and Lease Finance Receivables and Allowance for Loan Losses - Schedule of Recorded Investment in Loans Held-for-Investment and Related Allowance for Loan Losses by Loan Type (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment in Loans, Individually Evaluated for Impairment	$ 20,183		$ 17,308
Recorded Investment in Loans, Collectively Evaluated for Impairment	7,550,280		4,699,018
Recorded Investment in Loans, Acquired with Deterioriated Credit Quality	17,260		36,548
Allowance for Loan Losses, Individually Evaluated for Impairment	83		88
Allowance for Loan Losses, Collectively Evaluated for Impairment	59,719		60,112
Allowance for Loan Losses, Acquired with Deterioriated Credit Quality	205		431
PCI Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment in Loans, Individually Evaluated for Impairment	0		0
Recorded Investment in Loans, Collectively Evaluated for Impairment	0		0
Recorded Investment in Loans, Acquired with Deterioriated Credit Quality	17,260	$ 28,882	36,548
Allowance for Loan Losses, Individually Evaluated for Impairment	0		0
Allowance for Loan Losses, Collectively Evaluated for Impairment	0		0
Allowance for Loan Losses, Acquired with Deterioriated Credit Quality	205	$ 367	431
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment in Loans, Individually Evaluated for Impairment	3,168		745
Recorded Investment in Loans, Collectively Evaluated for Impairment	1,018,738		527,914
Recorded Investment in Loans, Acquired with Deterioriated Credit Quality	0		0
Allowance for Loan Losses, Individually Evaluated for Impairment	0		2
Allowance for Loan Losses, Collectively Evaluated for Impairment	7,491		8,061
Allowance for Loan Losses, Acquired with Deterioriated Credit Quality	0		0
Small Business Administration [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment in Loans, Individually Evaluated for Impairment	3,593		2,273
Recorded Investment in Loans, Collectively Evaluated for Impairment	353,459		121,818
Recorded Investment in Loans, Acquired with Deterioriated Credit Quality	0		0
Allowance for Loan Losses, Individually Evaluated for Impairment	0		3
Allowance for Loan Losses, Collectively Evaluated for Impairment	958		861
Allowance for Loan Losses, Acquired with Deterioriated Credit Quality	0		0
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment in Loans, Individually Evaluated for Impairment	6,348		8,168
Recorded Investment in Loans, Collectively Evaluated for Impairment	5,262,392		3,324,349
Recorded Investment in Loans, Acquired with Deterioriated Credit Quality	0		0
Allowance for Loan Losses, Individually Evaluated for Impairment	0		0
Allowance for Loan Losses, Collectively Evaluated for Impairment	41,541		40,870
Allowance for Loan Losses, Acquired with Deterioriated Credit Quality	0		0
Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment in Loans, Individually Evaluated for Impairment	0		0
Recorded Investment in Loans, Collectively Evaluated for Impairment	123,274		74,148
Recorded Investment in Loans, Acquired with Deterioriated Credit Quality	0		0
Allowance for Loan Losses, Individually Evaluated for Impairment	0		0
Allowance for Loan Losses, Collectively Evaluated for Impairment	1,133		933
Allowance for Loan Losses, Acquired with Deterioriated Credit Quality	0		0
SFR Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment in Loans, Individually Evaluated for Impairment	5,492		4,550
Recorded Investment in Loans, Collectively Evaluated for Impairment	287,024		240,112
Recorded Investment in Loans, Acquired with Deterioriated Credit Quality	0		0
Allowance for Loan Losses, Individually Evaluated for Impairment	13		0
Allowance for Loan Losses, Collectively Evaluated for Impairment	2,112		2,320
Allowance for Loan Losses, Acquired with Deterioriated Credit Quality	0		0
Dairy & Livestock and Agribusiness [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment in Loans, Individually Evaluated for Impairment	775		829
Recorded Investment in Loans, Collectively Evaluated for Impairment	303,823		269,653
Recorded Investment in Loans, Acquired with Deterioriated Credit Quality	0		0
Allowance for Loan Losses, Individually Evaluated for Impairment	0		0
Allowance for Loan Losses, Collectively Evaluated for Impairment	5,024		5,374
Allowance for Loan Losses, Acquired with Deterioriated Credit Quality	0		0
Municipal Lease Finance Receivables [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment in Loans, Individually Evaluated for Impairment	0		0
Recorded Investment in Loans, Collectively Evaluated for Impairment	67,581		71,352
Recorded Investment in Loans, Acquired with Deterioriated Credit Quality	0		0
Allowance for Loan Losses, Individually Evaluated for Impairment	0		0
Allowance for Loan Losses, Collectively Evaluated for Impairment	815		906
Allowance for Loan Losses, Acquired with Deterioriated Credit Quality	0		0
Consumer and Other Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment in Loans, Individually Evaluated for Impairment	807		743
Recorded Investment in Loans, Collectively Evaluated for Impairment	133,989		69,672
Recorded Investment in Loans, Acquired with Deterioriated Credit Quality	0		0
Allowance for Loan Losses, Individually Evaluated for Impairment	70		83
Allowance for Loan Losses, Collectively Evaluated for Impairment	645		787
Allowance for Loan Losses, Acquired with Deterioriated Credit Quality	$ 0		$ 0