Allowance for Probable Loan Losses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Allowance for probable loan losses
Balance at the beginning of the period	$ 84,192	$ 84,482
Losses charged to allowance	(12,084)	(4,469)
Recoveries credited to allowance	1,388	1,304
Net losses charged to allowance	(10,696)	(3,165)
Provision charged to operations	5,285	4,080
Balance at the end of the period	78,781	85,397
Commercial
Allowance for probable loan losses
Balance at the beginning of the period	26,617	22,046
Losses charged to allowance	(3,424)	(327)
Recoveries credited to allowance	1,244	121
Net losses charged to allowance	(2,180)	(206)
Provision charged to operations	140	1,801
Balance at the end of the period	24,577	23,641
Commercial real estate: other construction and land development
Allowance for probable loan losses
Balance at the beginning of the period	19,940	26,695
Losses charged to allowance	(71)	(941)
Recoveries credited to allowance	5	3
Net losses charged to allowance	(66)	(938)
Provision charged to operations	(108)	5,714
Balance at the end of the period	19,766	31,471
Commercial real estate: farmland and commercial
Allowance for probable loan losses
Balance at the beginning of the period	24,227	16,340
Losses charged to allowance	(7,994)
Recoveries credited to allowance	31	134
Net losses charged to allowance	(7,963)	134
Provision charged to operations	5,546	2,088
Balance at the end of the period	21,810	18,562
Commercial real estate: multifamily
Allowance for probable loan losses
Balance at the beginning of the period	1,003	53
Provision charged to operations	(154)	178
Balance at the end of the period	849	231
Residential: first lien
Allowance for probable loan losses
Balance at the beginning of the period	4,562	10,059
Losses charged to allowance	(36)	(21)
Recoveries credited to allowance	2	2
Net losses charged to allowance	(34)	(19)
Provision charged to operations	(149)	(5,599)
Balance at the end of the period	4,379	4,441
Residential: junior lien
Allowance for probable loan losses
Balance at the beginning of the period	4,760	2,611
Losses charged to allowance	(312)	(235)
Recoveries credited to allowance	45	197
Net losses charged to allowance	(267)	(38)
Provision charged to operations	12	1,700
Balance at the end of the period	4,505	4,273
Consumer
Allowance for probable loan losses
Balance at the beginning of the period	1,724	6,241
Losses charged to allowance	(247)	(2,936)
Recoveries credited to allowance	61	847
Net losses charged to allowance	(186)	(2,089)
Provision charged to operations	64	(1,942)
Balance at the end of the period	1,602	2,210
Foreign
Allowance for probable loan losses
Balance at the beginning of the period	1,359	437
Losses charged to allowance		(9)
Net losses charged to allowance		(9)
Provision charged to operations	66	140
Balance at the end of the period	$ 1,293	$ 568