Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended	12 Months Ended								9 Months Ended	0 Months Ended	63 Months Ended		9 Months Ended
	Mar. 31, 2013	Jun. 30, 2009	Sep. 30, 2014 Segment Trust	Dec. 31, 2013	Jun. 30, 2014	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Change in Accounting Method Accounted for as Change in Estimate [Member]	Jun. 30, 2013 Impaired real estate collateral dependent loans [Member]	Mar. 31, 2013 Impaired real estate collateral dependent loans [Member]	Sep. 30, 2014 California [Member] Business	Oct. 03, 2014 Subsequent event [Member] Tri Counties Bank [Member] Location Branches	Sep. 30, 2014 Minimum [Member]	Sep. 30, 2014 Maximum [Member]	Sep. 30, 2014 Furniture and equipment [Member] Minimum [Member]	Sep. 30, 2014 Furniture and equipment [Member] Maximum [Member]	Sep. 30, 2014 Land improvements and buildings [Member] Minimum [Member]	Sep. 30, 2014 Land improvements and buildings [Member] Maximum [Member]
Significant Of Accounting Policies [Line Items]
Number of counties												26
Number of locations acquired													22
Number of traditional operating branches													61
Number of in-store operating branches													20
Number of subsidiary business trusts			2
Number of business segment			1
Investment securities held to maturity	$ 0		$ 443,509,000	$ 240,504,000
Trading securities			0	0
Recognized OTTI losses			0	0
Loans contractual past due			90 days
TDR loans payment period in nonaccrual status			6 months
Method used to estimate net sale proceeds	Greater of fifteen percent or actual estimated selling costs.
Allowance for loan losses			37,920,000	38,245,000	39,968,000	39,340,000	39,599,000	42,648,000	1,665,000	1,314,000	494,000
Allowance for loss factor weighted percentage		75.00%	125.00%											75.00%	125.00%
Reserve for unimpaired loans					1,438,000
Estimated useful lives of the related assets or lease terms																3 years	10 years	15 years	40 years
Reserve for unfunded commitments due to change in methodology							$ 335,000