Loans and Allowance for Probable Loan Losses - Aging of Past Due Loans by Class of Loan (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Abstract]
30-59 Days Past Due	$ 15,768		$ 5,849
60-89 Days Past Due	1,320		620
Greater than 90 Days Past Due	1,090		4,100
Total Past Due	18,178		10,569
Current	2,156,436	[1]	2,170,564	[1]
Total ending loans balance	2,174,614	[2]	2,181,133	[2]
Construction
Financing Receivable, Recorded Investment, Past Due [Abstract]
30-59 Days Past Due	1,777		376
60-89 Days Past Due	0		42
Greater than 90 Days Past Due	118		716
Total Past Due	1,895		1,134
Current	235,341	[1]	242,352	[1]
Total ending loans balance	237,236		243,486
1-4 Family Residential
Financing Receivable, Recorded Investment, Past Due [Abstract]
30-59 Days Past Due	6,329		3,511
60-89 Days Past Due	589		509
Greater than 90 Days Past Due	306		2,017
Total Past Due	7,224		6,037
Current	684,731	[1]	683,251	[1]
Total ending loans balance	691,955		689,288
Other
Financing Receivable, Recorded Investment, Past Due [Abstract]
30-59 Days Past Due	1,793		1,203
60-89 Days Past Due	54		0
Greater than 90 Days Past Due	179		675
Total Past Due	2,026		1,878
Current	500,450	[1]	483,348	[1]
Total ending loans balance	502,476		485,226
Commercial Loans
Financing Receivable, Recorded Investment, Past Due [Abstract]
30-59 Days Past Due	132		397
60-89 Days Past Due	254		3
Greater than 90 Days Past Due	72		416
Total Past Due	458		816
Current	248,949	[1]	234,540	[1]
Total ending loans balance	249,407		235,356
Municipal Loans
Financing Receivable, Recorded Investment, Past Due [Abstract]
30-59 Days Past Due	2,763		0
60-89 Days Past Due	0		0
Greater than 90 Days Past Due	0		0
Total Past Due	2,763		0
Current	249,993	[1]	257,492	[1]
Total ending loans balance	252,756		257,492
Loans to Individuals
Financing Receivable, Recorded Investment, Past Due [Abstract]
30-59 Days Past Due	2,974		362
60-89 Days Past Due	423		66
Greater than 90 Days Past Due	415		276
Total Past Due	3,812		704
Current	236,972	[1]	269,581	[1]
Total ending loans balance	$ 240,784		$ 270,285

[1] Includes PCI loans measured at fair value at acquisition.

[2] Includes approximately $727.7 million and $763.3 million of loans acquired with the Omni acquisition as of March 31, 2015 and December 31, 2014, respectively. These loans were measured at fair value at the acquisition date with no carryover of allowance for loan loss.