Acquisitions - Additional Information (Detail) $ in Thousands				3 Months Ended					6 Months Ended
	Feb. 10, 2015 USD ($) Office shares	May. 16, 2014 USD ($) County Office shares	Mar. 05, 2014 USD ($) Office	Jun. 30, 2015 USD ($) Office	Mar. 31, 2015 USD ($)	Dec. 31, 2014 Office	Jun. 30, 2014 USD ($) Office	Mar. 31, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)
Schedule Of Compensation Cost For Share Based Payment Arrangements Allocation of Share Based Compensation Cost By Income Statement [Line Items]
Net interest income | $				$ 93,756			$ 64,801		$ 179,246	$ 117,198
Net income | $				$ 44,804			$ 26,478		84,722	$ 51,746
Number of banking offices closed				8		8
Summit Bancorp, Inc. [Member]
Schedule Of Compensation Cost For Share Based Payment Arrangements Allocation of Share Based Compensation Cost By Income Statement [Line Items]
Number of common stock issued | shares		5,765,846
Date of acquisition		May 16, 2014
Goodwill | $		$ 73,400
Cash consideration | $		$ 42,500
Number of banking offices closed				6		6	1
Number of loan production office closed							1
Intervest Bancshares Corporation [Member]
Schedule Of Compensation Cost For Share Based Payment Arrangements Allocation of Share Based Compensation Cost By Income Statement [Line Items]
Goodwill period decrease | $					$ 2,700
Goodwill | $				$ 44,200					44,200
Net interest income | $				14,900					23,800
Net income | $				$ 8,600					$ 13,500
Bancshares Inc [Member]
Schedule Of Compensation Cost For Share Based Payment Arrangements Allocation of Share Based Compensation Cost By Income Statement [Line Items]
Date of acquisition			Mar. 05, 2014
Cash consideration | $			$ 21,500
Bargain purchase gain recognized amount | $								$ 4,700
Intervest [Member]
Schedule Of Compensation Cost For Share Based Payment Arrangements Allocation of Share Based Compensation Cost By Income Statement [Line Items]
Number of common stock issued | shares	6,637,243
Total consideration | $	$ 238,500
Date of acquisition									Feb. 10, 2015
Central South And Western Arkansas [Member] | Summit Bancorp, Inc. [Member]
Schedule Of Compensation Cost For Share Based Payment Arrangements Allocation of Share Based Compensation Cost By Income Statement [Line Items]
Number of bank offices		23
Arkansas Counties [Member] | Summit Bancorp, Inc. [Member]
Schedule Of Compensation Cost For Share Based Payment Arrangements Allocation of Share Based Compensation Cost By Income Statement [Line Items]
Number of loan production office		1
Number of Arkansas countries | County		9
Clearwater [Member] | Intervest National Bank (INB) [Member]
Schedule Of Compensation Cost For Share Based Payment Arrangements Allocation of Share Based Compensation Cost By Income Statement [Line Items]
Number of bank offices	5
Pasadena [Member] | Intervest National Bank (INB) [Member]
Schedule Of Compensation Cost For Share Based Payment Arrangements Allocation of Share Based Compensation Cost By Income Statement [Line Items]
Number of bank offices	1
New York [Member] | Intervest National Bank (INB) [Member]
Schedule Of Compensation Cost For Share Based Payment Arrangements Allocation of Share Based Compensation Cost By Income Statement [Line Items]
Number of bank offices	1
Houston [Member] | Bancshares Inc [Member]
Schedule Of Compensation Cost For Share Based Payment Arrangements Allocation of Share Based Compensation Cost By Income Statement [Line Items]
Number of bank offices			3
Austin [Member] | Bancshares Inc [Member]
Schedule Of Compensation Cost For Share Based Payment Arrangements Allocation of Share Based Compensation Cost By Income Statement [Line Items]
Number of bank offices			1
Cedar Park [Member] | Bancshares Inc [Member]
Schedule Of Compensation Cost For Share Based Payment Arrangements Allocation of Share Based Compensation Cost By Income Statement [Line Items]
Number of bank offices			1
Lockhart [Member] | Bancshares Inc [Member]
Schedule Of Compensation Cost For Share Based Payment Arrangements Allocation of Share Based Compensation Cost By Income Statement [Line Items]
Number of bank offices			1
San Antonio [Member] | Bancshares Inc [Member]
Schedule Of Compensation Cost For Share Based Payment Arrangements Allocation of Share Based Compensation Cost By Income Statement [Line Items]
Number of bank offices			1