Allowance for Probable Loan Losses (Details 4) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Interest income recognized:
Loans accounted for as troubled debt restructuring	$ 19,232,000	$ 24,300,000
Period of charge off for past due unsecured commercial loans	90 days
Class of financing receivable
Recorded investment:
Total impaired loans with related allowance	30,622,000	11,982,000
Total impaired loans with no related allowance	48,851,000	71,023,000
Unpaid principal balance:
Total impaired loans with an allowance recorded	34,024,000	15,273,000
Total impaired loans with no related allowance recorded	49,828,000	73,728,000
Related Allowance	15,011,000	4,746,000
Average recorded investment:
Total impaired loans with an allowance recorded	30,618,000	35,076,000
Total impaired loans with no related allowance recorded	48,651,000	65,536,000
Interest income recognized:
Total impaired loans with an allowance recorded	32,000	131,000
Total impaired loans with no related allowance recorded	81,000	323,000
Commercial | Class of financing receivable
Recorded investment:
Total impaired loans with related allowance	18,668,000	1,633,000
Total impaired loans with no related allowance	14,440,000	31,135,000
Unpaid principal balance:
Total impaired loans with an allowance recorded	18,702,000	1,679,000
Total impaired loans with no related allowance recorded	14,487,000	31,170,000
Related Allowance	11,544,000	1,477,000
Average recorded investment:
Total impaired loans with an allowance recorded	18,688,000	21,126,000
Total impaired loans with no related allowance recorded	15,147,000	2,996,000
Interest income recognized:
Total impaired loans with an allowance recorded	9,000	39,000
Total impaired loans with no related allowance recorded	1,000	4,000
Commercial real estate: other construction and land development | Class of financing receivable
Recorded investment:
Total impaired loans with related allowance	5,516,000	3,671,000
Total impaired loans with no related allowance	20,656,000	24,989,000
Unpaid principal balance:
Total impaired loans with an allowance recorded	5,605,000	3,671,000
Total impaired loans with no related allowance recorded	20,743,000	25,160,000
Related Allowance	823,000	539,000
Average recorded investment:
Total impaired loans with an allowance recorded	5,478,000	6,608,000
Total impaired loans with no related allowance recorded	20,878,000	39,449,000
Interest income recognized:
Total impaired loans with no related allowance recorded	18,000	141,000
Commercial real estate: farmland and commercial | Class of financing receivable
Recorded investment:
Total impaired loans with related allowance	6,438,000	6,678,000
Total impaired loans with no related allowance	5,462,000	7,267,000
Unpaid principal balance:
Total impaired loans with an allowance recorded	9,717,000	9,923,000
Total impaired loans with no related allowance recorded	6,232,000	9,340,000
Related Allowance	2,644,000	2,730,000
Average recorded investment:
Total impaired loans with an allowance recorded	6,452,000	7,342,000
Total impaired loans with no related allowance recorded	5,006,000	16,536,000
Interest income recognized:
Total impaired loans with an allowance recorded	23,000	92,000
Total impaired loans with no related allowance recorded		8,000
Commercial real estate: multifamily | Class of financing receivable
Recorded investment:
Total impaired loans with no related allowance	338,000	353,000
Unpaid principal balance:
Total impaired loans with no related allowance recorded	338,000	353,000
Average recorded investment:
Total impaired loans with no related allowance recorded	345,000	381,000
Residential: first lien | Class of financing receivable
Recorded investment:
Total impaired loans with no related allowance	4,546,000	3,656,000
Unpaid principal balance:
Total impaired loans with no related allowance recorded	4,595,000	3,984,000
Average recorded investment:
Total impaired loans with no related allowance recorded	3,840,000	2,876,000
Interest income recognized:
Total impaired loans with no related allowance recorded	33,000	60,000
Residential: junior lien | Class of financing receivable
Recorded investment:
Total impaired loans with no related allowance	1,702,000	1,850,000
Unpaid principal balance:
Total impaired loans with no related allowance recorded	1,720,000	1,944,000
Average recorded investment:
Total impaired loans with no related allowance recorded	1,732,000	1,939,000
Interest income recognized:
Total impaired loans with no related allowance recorded	24,000	104,000
Consumer | Class of financing receivable
Recorded investment:
Total impaired loans with no related allowance	1,245,000	1,326,000
Unpaid principal balance:
Total impaired loans with no related allowance recorded	1,250,000	1,330,000
Average recorded investment:
Total impaired loans with no related allowance recorded	1,246,000	1,193,000
Foreign | Class of financing receivable
Recorded investment:
Total impaired loans with no related allowance	462,000	447,000
Unpaid principal balance:
Total impaired loans with no related allowance recorded	463,000	447,000
Average recorded investment:
Total impaired loans with no related allowance recorded	457,000	166,000
Interest income recognized:
Total impaired loans with no related allowance recorded	$ 5,000	$ 6,000