Allowance for Probable Loan Losses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Allowance for probable loan losses
Balance at the beginning of the period	$ 58,193	$ 84,192
Losses charged to allowance	(3,509)	(12,084)
Recoveries credited to allowance	865	1,388
Net losses charged to allowance	(2,644)	(10,696)
Provision charged to operations	7,419	5,285
Balance at the end of the period	62,968	78,781
Increase in allowance for probable loan losses (as a percent)	8.20%
Addition of a specific reserve on a previously identified impaired loan	10,000
Commercial
Allowance for probable loan losses
Balance at the beginning of the period	11,632	26,617
Losses charged to allowance	(2,663)	(3,424)
Recoveries credited to allowance	694	1,244
Net losses charged to allowance	(1,969)	(2,180)
Provision charged to operations	11,263	140
Balance at the end of the period	20,926	24,577
Commercial real estate: other construction and land development
Allowance for probable loan losses
Balance at the beginning of the period	12,720	19,940
Losses charged to allowance	(128)	(71)
Recoveries credited to allowance	13	5
Net losses charged to allowance	(115)	(66)
Provision charged to operations	(1,552)	(108)
Balance at the end of the period	11,053	19,766
Commercial real estate: farmland and commercial
Allowance for probable loan losses
Balance at the beginning of the period	21,880	24,227
Losses charged to allowance	(60)	(7,994)
Recoveries credited to allowance	13	31
Net losses charged to allowance	(47)	(7,963)
Provision charged to operations	(1,182)	5,546
Balance at the end of the period	20,651	21,810
Commercial real estate: multifamily
Allowance for probable loan losses
Balance at the beginning of the period	694	1,003
Provision charged to operations	(85)	(154)
Balance at the end of the period	609	849
Residential: first lien
Allowance for probable loan losses
Balance at the beginning of the period	4,390	4,562
Losses charged to allowance	(172)	(36)
Recoveries credited to allowance	5	2
Net losses charged to allowance	(167)	(34)
Provision charged to operations	(355)	(149)
Balance at the end of the period	3,868	4,379
Residential: junior lien
Allowance for probable loan losses
Balance at the beginning of the period	4,448	4,760
Losses charged to allowance	(255)	(312)
Recoveries credited to allowance	94	45
Net losses charged to allowance	(161)	(267)
Provision charged to operations	(276)	12
Balance at the end of the period	4,011	4,505
Consumer
Allowance for probable loan losses
Balance at the beginning of the period	1,289	1,724
Losses charged to allowance	(211)	(247)
Recoveries credited to allowance	46	61
Net losses charged to allowance	(165)	(186)
Provision charged to operations	(304)	64
Balance at the end of the period	820	1,602
Foreign
Allowance for probable loan losses
Balance at the beginning of the period	1,140	1,359
Losses charged to allowance	(20)
Net losses charged to allowance	(20)
Provision charged to operations	(90)	(66)
Balance at the end of the period	$ 1,030	$ 1,293