Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended	12 Months Ended				60 Months Ended		6 Months Ended		6 Months Ended
	Mar. 31, 2013	Jun. 30, 2009	Jun. 30, 2014 Trust Segment	Dec. 31, 2013	Mar. 31, 2014	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2014 Minimum [Member]	Jun. 30, 2014 Maximum [Member]	Jun. 30, 2014 Tri Counties Bank [Member] Branches	Sep. 30, 2013 Change in Accounting Method Accounted for as Change in Estimate [Member]	Jun. 30, 2014 Furniture and equipment [Member] Minimum [Member]	Jun. 30, 2014 Furniture and equipment [Member] Maximum [Member]	Jun. 30, 2014 Land improvements and buildings [Member] Minimum [Member]	Jun. 30, 2014 Land improvements and buildings [Member] Maximum [Member]	Jun. 30, 2013 Impaired real estate collateral dependent loans [Member]	Mar. 31, 2013 Impaired real estate collateral dependent loans [Member]
Significant Of Accounting Policies [Line Items]
Number of traditional operating branches										41
Number of in-store operating branches										19
Number of subsidiary business trusts			2
Number of business segment			1
Investment securities held to maturity	$ 0		$ 422,502,000	$ 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	39,867,000		39,968,000	38,245,000	38,322,000	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