Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) Office Segment Location Trust Branch Business	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)
Significant Of Accounting Policies [Line Items]
Number of subsidiary business trusts | Trust	5
Number of loan production offices | Office	2
Company's investments in the trusts	$ 1,703,000
Number of business segment | Segment	1
Marketable securities recorded	$ 2,890,000		$ 2,938,000
Unrealized losses recognized in condensed consolidated statements of income	48,000
OTTI losses due to debt securities	$ 0		$ 0
Loans contractual past due	90 days
TDR loans payment period in nonaccrual status	6 months
Federal statutory income tax rate	21.00%	35.00%	35.00%
Tri Counties Bank [Member]
Significant Of Accounting Policies [Line Items]
Number of locations acquired | Location	3
Number of traditional operating branches | Branch	57
Number of in-store operating branches | Branch	9
Trading Debt Securities [Member]
Significant Of Accounting Policies [Line Items]
Trading debt securities	$ 0		$ 0
California [Member]
Significant Of Accounting Policies [Line Items]
Number of counties | Business	26
ASU 2016-01 [Member] | Deferred Tax Assets [Member]
Significant Of Accounting Policies [Line Items]
New accounting pronouncement or change in accounting principle cumulative effect of change on equity or net assets				$ (18,000)
ASU 2016-01 [Member] | Retained Earnings [Member]
Significant Of Accounting Policies [Line Items]
New accounting pronouncement or change in accounting principle cumulative effect of change on equity or net assets				(62,000)
ASU 2016-09 [Member]
Significant Of Accounting Policies [Line Items]
Recognition of excess tax benefits		$ 90,000
ASU 2018-02 [Member] | Retained Earnings [Member]
Significant Of Accounting Policies [Line Items]
New accounting pronouncement or change in accounting principle cumulative effect of change on equity or net assets				1,093,000
ASU 2018-02 [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Significant Of Accounting Policies [Line Items]
New accounting pronouncement or change in accounting principle cumulative effect of change on equity or net assets				$ (1,093,000)
Furniture and Equipment [Member] | Minimum [Member]
Significant Of Accounting Policies [Line Items]
Estimated useful lives of the related assets or lease terms	3 years
Furniture and Equipment [Member] | Maximum [Member]
Significant Of Accounting Policies [Line Items]
Estimated useful lives of the related assets or lease terms	10 years
Land Improvements and Buildings [Member] | Minimum [Member]
Significant Of Accounting Policies [Line Items]
Estimated useful lives of the related assets or lease terms	15 years
Land Improvements and Buildings [Member] | Maximum [Member]
Significant Of Accounting Policies [Line Items]
Estimated useful lives of the related assets or lease terms	40 years