Fair Value of Financial Instrument - Carrying Amount and Estimated Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Financial assets:
Cash and due from banks	$ 115,413	$ 78,036	$ 80,986	$ 62,955
Federal funds sold and interest-bearing accounts	239,804	92,323	44,800
FDIC loss-share receivable	14,957	31,351	49,180	$ 65,441
Financial liabilities:
Other borrowings	39,000	78,881	100,293
Subordinated deferrable interest debentures	69,325	65,325	64,842
Carrying Value [Member]
Financial assets:
Cash and due from banks	115,413	78,026	80,986
Federal funds sold and interest-bearing accounts	239,804	92,323	44,800
Loans, net	3,407,609	2,783,763	2,745,897
FDIC loss-share receivable	14,957	31,351	49,180
Accrued interest receivable	17,648	17,023	15,711
Financial liabilities:
Deposits	4,511,547	3,431,149	3,389,035
Securities sold under agreements to repurchase	75,066	73,310	51,109
Other borrowings	39,000	78,881	100,293
Accrued interest payable	1,239	1,382	1,423
Subordinated deferrable interest debentures	69,325	65,325	64,842
Fair Value [Member]
Financial assets:
Cash and due from banks	115,413	78,026	80,986
Federal funds sold and interest-bearing accounts	239,804	92,323	44,800
Loans, net	3,428,008	2,785,627	2,754,953
FDIC loss-share receivable	5,295	18,764	36,182
Accrued interest receivable	17,648	17,023	15,711
Financial liabilities:
Deposits	4,513,218	3,432,059	3,389,880
Securities sold under agreements to repurchase	75,066	73,310	51,109
Other borrowings	39,000	78,881	100,293
Accrued interest payable	1,239	1,382	1,423
Subordinated deferrable interest debentures	50,924	46,564	45,864
Fair Value [Member] | Level 1 [Member]
Financial assets:
Cash and due from banks	115,413	78,026	80,986
Federal funds sold and interest-bearing accounts	239,804	92,323	44,800
Accrued interest receivable	17,648	17,023	15,711
Financial liabilities:
Securities sold under agreements to repurchase	75,066	73,310	51,109
Accrued interest payable	1,239	1,382	1,423
Fair Value [Member] | Level 2 [Member]
Financial liabilities:
Deposits	4,513,218	3,432,059	3,389,880
Other borrowings	39,000	78,881	100,293
Subordinated deferrable interest debentures	50,924	46,564	45,864
Fair Value [Member] | Level 3 [Member]
Financial assets:
Loans, net	3,428,008	2,785,627	2,754,953
FDIC loss-share receivable	$ 5,295	$ 18,764	$ 36,182