Loans and Lease Finance Receivables and Allowance for Loan Losses - Summary of Loan, Excluding PCI Loans by Internal Risk Ratings (Detail) - Loans, Excluding PCI Loans [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	$ 1,021,906	$ 513,325
SBA	357,052	122,055
Commercial real estate, Owner occupied	2,041,131	1,096,280
Commercial real estate, Non-owner occupied	3,227,609	2,280,433
Construction, Speculative	32,081	60,042
Construction, Non-speculative	91,193	17,940
SFR mortgage	292,516	236,202
Dairy & livestock and agribusiness	304,598	347,289
Municipal lease finance receivables	67,581	70,243
Consumer and other loans	134,796	64,229
Total gross loans, excluding PCI loans	7,570,463	4,808,038
Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	980,421	483,641
SBA	345,126	112,835
Commercial real estate, Owner occupied	1,931,062	1,009,199
Commercial real estate, Non-owner occupied	3,215,070	2,257,130
Construction, Speculative	32,081	60,042
Construction, Non-speculative	91,193	17,940
SFR mortgage	284,852	229,032
Dairy & livestock and agribusiness	268,328	321,413
Municipal lease finance receivables	67,045	69,644
Consumer and other loans	132,637	61,715
Total gross loans, excluding PCI loans	7,347,815	4,622,591
Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	33,628	19,566
SBA	5,469	5,358
Commercial real estate, Owner occupied	97,990	76,111
Commercial real estate, Non-owner occupied	5,582	16,434
Construction, Speculative	0	0
Construction, Non-speculative	0	0
SFR mortgage	4,047	3,124
Dairy & livestock and agribusiness	26,877	9,047
Municipal lease finance receivables	536	599
Consumer and other loans	740	1,255
Total gross loans, excluding PCI loans	174,869	131,494
Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	7,857	10,118
SBA	6,457	3,862
Commercial real estate, Owner occupied	12,079	10,970
Commercial real estate, Non-owner occupied	6,957	6,869
Construction, Speculative	0	0
Construction, Non-speculative	0	0
SFR mortgage	3,617	4,046
Dairy & livestock and agribusiness	9,393	16,829
Municipal lease finance receivables	0	0
Consumer and other loans	1,419	1,259
Total gross loans, excluding PCI loans	47,779	53,953
Doubtful & Loss [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and industrial	0	0
SBA	0	0
Commercial real estate, Owner occupied	0	0
Commercial real estate, Non-owner occupied	0	0
Construction, Speculative	0	0
Construction, Non-speculative	0	0
SFR mortgage	0	0
Dairy & livestock and agribusiness	0	0
Municipal lease finance receivables	0	0
Consumer and other loans	0	0
Total gross loans, excluding PCI loans	$ 0	$ 0