Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	12 Months Ended	0 Months Ended		3 Months Ended
	Dec. 31, 2014 Subsidiary segment	Mar. 09, 2012 Subsidiary Branch	Aug. 12, 2013 Branch	Jun. 30, 2015	Dec. 31, 2013	Oct. 29, 2014 branch
Summary Of Significant Accounting Policies [Line Items]
Number of Reporting Units	3
Number of subsidiary	3
Percentage of outstanding common stock of investees accounted for using equity method of accounting	100.00%
Period for satisfactory payment of TDRs	6 months
Evaluation for impairment of substandard and nonaccrual commercial loans	$ 500,000
Carrying amount of goodwill	175,820,000				175,820,000
Amortization period of intangible assets	10 years
Percentage of amount recognized of tax benefit	50.00%
Percentage of projected benefit obligation	10.00%
Assets	4,964,686,000				4,533,190,000
Community Banking [Member]
Summary Of Significant Accounting Policies [Line Items]
Carrying amount of goodwill	171,600,000
Percentage of goodwill to reporting units	98.00%
Assets	4,954,728,000				4,524,939,000
Insurance [Member]
Summary Of Significant Accounting Policies [Line Items]
Carrying amount of goodwill	4,200,000
Percentage of goodwill to reporting units	2.00%
Assets	7,468,000				6,926,000
Common Wealth Trust Life Insurance Company [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of outstanding common stock of investees accounted for using equity method of accounting	50.00%
Mainline [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of subsidiary		1
Business acquisition date	Mar. 09, 2012
Number of branches		8
Business acquisition, loans		129,500,000
Business acquisition, deposits		206,000,000
Business acquisition cost		27,800,000
Gateway [Member]
Summary Of Significant Accounting Policies [Line Items]
Business acquisition date	Aug. 13, 2012
Number of branches			2
Business acquisition, loans			99,100,000
Business acquisition, deposits			105,400,000
Business acquisition cost			19,800,000
Integrity Bancshares, Inc. [Member]
Summary Of Significant Accounting Policies [Line Items]
Transaction value						155,000,000
Number of Branches						8
Assets						$ 860,000,000
Interest Rate Lock Commitments [Member]
Summary Of Significant Accounting Policies [Line Items]
Period for interest rate lock commitment	60 days
Commercial Real Estate [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for loan loss emergence period	3 years 6 months
Period lengthened the LEP assumption for the consumer portfolio	1 year 6 months
Commercial and Industrial [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for loan loss emergence period	2 years 6 months
Consumer Portfolio Segment [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for loan loss emergence period	2 years
Period lengthened the LEP assumption for the consumer portfolio	1 year 6 months
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of outstanding common stock of investees accounted for using equity method of accounting	20.00%
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of outstanding common stock of investees accounted for using equity method of accounting	50.00%
Scenario, Forecast [Member] | Integrity Bancshares, Inc. [Member]
Summary Of Significant Accounting Policies [Line Items]
Period of intended operation subsequent to merger				3 years