Loans and Lease Finance Receivables and Allowance for Loan Losses - Schedule of Balance and Activity Related to Allowance for Loan Losses for Held-for-Investment Loans by Type (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning Balance	$ 59,583	$ 60,201	$ 59,585	$ 61,540
Charge-offs	(258)	(147)	(267)	(149)
Recoveries	182	2,077	2,189	6,240
(Recapture of) provision for loan losses	500	(1,500)	(1,500)	(7,000)
Ending Balance	60,007	60,631	60,007	60,631
PCI Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning Balance	216	659	367	1,219
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
(Recapture of) provision for loan losses	(11)	(228)	(162)	(788)
Ending Balance	205	431	205	431
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning Balance	6,970	8,060	7,280	8,154
Charge-offs	0	(138)	0	(138)
Recoveries	44	12	81	106
(Recapture of) provision for loan losses	477	129	130	(59)
Ending Balance	7,491	8,063	7,491	8,063
Small Business Administration [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning Balance	841	913	869	871
Charge-offs	(257)	0	(257)	0
Recoveries	5	5	15	47
(Recapture of) provision for loan losses	369	(54)	331	(54)
Ending Balance	958	864	958	864
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning Balance	42,597	39,927	41,722	37,443
Charge-offs	0	0	0	0
Recoveries	0	0	0	154
(Recapture of) provision for loan losses	(1,056)	943	(181)	3,273
Ending Balance	41,541	40,870	41,541	40,870
Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning Balance	1,003	1,059	984	1,096
Charge-offs	0	0	0	0
Recoveries	15	2,055	1,945	5,774
(Recapture of) provision for loan losses	115	(2,181)	(1,796)	(5,937)
Ending Balance	1,133	933	1,133	933
SFR Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning Balance	2,155	2,369	2,112	2,287
Charge-offs	0	0	0	0
Recoveries	0	0	0	64
(Recapture of) provision for loan losses	(30)	(49)	13	(31)
Ending Balance	2,125	2,320	2,125	2,320
Dairy & Livestock and Agribusiness [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning Balance	4,351	5,440	4,647	8,541
Charge-offs	0	0	0	0
Recoveries	0	0	19	19
(Recapture of) provision for loan losses	673	(66)	358	(3,186)
Ending Balance	5,024	5,374	5,024	5,374
Municipal Lease Finance Receivables [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning Balance	808	852	851	941
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
(Recapture of) provision for loan losses	7	54	(36)	(35)
Ending Balance	815	906	815	906
Consumer and Other Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning Balance	642	922	753	988
Charge-offs	(1)	(9)	(10)	(11)
Recoveries	118	5	129	76
(Recapture of) provision for loan losses	(44)	(48)	(157)	(183)
Ending Balance	$ 715	$ 870	$ 715	$ 870