Loans and Loans Held for Sale - Additional Information (Detail) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2015 USD ($) SecurityLoan	Sep. 30, 2015 USD ($) SecurityLoan	Mar. 04, 2015 USD ($)	Dec. 31, 2014 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, unearned income	$ 2,700	$ 2,700		$ 2,100
Discount on loans related to purchase accounting fair value adjustments	$ 12,000	$ 12,000		$ 2,000
Percentage of commercial loans in total portfolio loans	76.00%	76.00%		75.00%
Commercial Loans	$ 3,733,828	$ 3,733,828		$ 2,892,522
Concentration Risk, Product		no
Maximum concentration of commercial real estate and commercial construction portfolio in loans (in excess of)	7.00%	7.00%		7.00%
Minimum period of loan payment defaults following restructure for TDRs to be in default		90 days
Commercial Real Estate and Commercial Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	$ 2,500,000	$ 2,500,000
Combined percentage of commercial real estate and commercial construction in total commercial loans	67.00%	67.00%		66.00%
Combined percentage of commercial real estate and commercial construction in total portfolio loans	51.00%	51.00%		49.00%
Out of market exposure of combined portfolio (percent)	6.40%	6.40%		8.00%
Percentage of total loans out-of-state excluding contiguous states	3.30%	3.30%		3.90%
Commercial Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	$ 384,328	$ 384,328		$ 216,148
Number Of Loans Modified Not Considered To Be Troubled Debt Restructuring | SecurityLoan	11	13
Loans modified not considered to be troubled debt restructuring	$ 7,100	$ 8,400
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	$ 2,111,585	$ 2,111,585		1,682,236
Number Of Loans Modified Not Considered To Be Troubled Debt Restructuring | SecurityLoan	3	6
Loans modified not considered to be troubled debt restructuring	$ 4,300	$ 5,300
Home Equity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number Of Loans Modified Not Considered To Be Troubled Debt Restructuring | SecurityLoan	1	3
Loans modified not considered to be troubled debt restructuring	$ 100	$ 300
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number Of Loans Modified Not Considered To Be Troubled Debt Restructuring | SecurityLoan	1	1
Loans modified not considered to be troubled debt restructuring	$ 100	$ 100
Integrity Bancshares, Inc. [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Discount on loans related to purchase accounting fair value adjustments			$ 14,800
Integrity Bancshares, Inc. [Member] | Commercial Real Estate and Commercial Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	424,000	424,000
Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	$ 1,237,915	$ 1,237,915		$ 994,138
Number Of Loans Modified Not Considered To Be Troubled Debt Restructuring | SecurityLoan	2	8
Loans modified not considered to be troubled debt restructuring	$ 1,100	$ 6,800