Loans and Allowance for Probable Loan Losses - Allowance for Loan Losses Activity by Portfolio Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015		Jun. 30, 2014
Change in Allowances for Loan Losses [Abstract]
Balance at beginning of period	$ 16,926	$ 18,787	$ 13,292	[1]	$ 18,877
Provision (reversal) for loan losses	268	2,650	4,116		6,783
Loans charged off	(893)	(3,546)	(1,979)		(8,314)
Recoveries of loans charged off	521	517	1,393		1,062
Balance at end of period	16,822	18,408	16,822		18,408
Construction
Change in Allowances for Loan Losses [Abstract]
Balance at beginning of period	2,774	2,130	2,456	[1]	2,142
Provision (reversal) for loan losses	86	319	361		309
Loans charged off	0	0	0		(14)
Recoveries of loans charged off	49	47	92		59
Balance at end of period	2,909	2,496	2,909		2,496
1-4 Family Residential
Change in Allowances for Loan Losses [Abstract]
Balance at beginning of period	3,400	3,797	2,822	[1]	3,277
Provision (reversal) for loan losses	93	92	666		628
Loans charged off	(40)	0	(46)		(22)
Recoveries of loans charged off	15	26	26		32
Balance at end of period	3,468	3,915	3,468		3,915
Other
Change in Allowances for Loan Losses [Abstract]
Balance at beginning of period	3,360	2,411	3,025	[1]	2,572
Provision (reversal) for loan losses	162	24	431		(140)
Loans charged off	0	0	0		0
Recoveries of loans charged off	7	1	73		4
Balance at end of period	3,529	2,436	3,529		2,436
Commercial Loans
Change in Allowances for Loan Losses [Abstract]
Balance at beginning of period	5,316	2,031	3,279	[1]	1,970
Provision (reversal) for loan losses	(308)	(322)	1,757		(319)
Loans charged off	(50)	(5)	(107)		(5)
Recoveries of loans charged off	55	53	84		111
Balance at end of period	5,013	1,757	5,013		1,757
Municipal Loans
Change in Allowances for Loan Losses [Abstract]
Balance at beginning of period	824	777	716	[1]	668
Provision (reversal) for loan losses	9	(114)	117		(5)
Loans charged off	0	0	0		0
Recoveries of loans charged off	0	0	0		0
Balance at end of period	833	663	833		663
Loans to Individuals
Change in Allowances for Loan Losses [Abstract]
Balance at beginning of period	1,252	7,641	994	[1]	8,248
Provision (reversal) for loan losses	226	2,651	784		6,310
Loans charged off	(803)	(3,541)	(1,826)		(8,273)
Recoveries of loans charged off	395	390	1,118		856
Balance at end of period	$ 1,070	$ 7,141	$ 1,070		$ 7,141

[1] Loans acquired with the Omni acquisition were measured at fair value on December 17, 2014 with no carryover of allowance for loan loss.