Allowance for Probable Loan Losses (By Loan Class) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Allowance for probable loan losses
Balance at the beginning of the period	$ 67,687	$ 64,661
Losses charged to allowance	(2,877)	(2,992)
Recoveries credited to allowance	682	817
Net (losses) recoveries charged to allowance	(2,195)	(2,175)
Provision charged to operations	1,662	1,700
Balance at the end of the period	$ 67,154	64,186
Period of time for determining the historical charge-off experience	3 years
Commercial
Allowance for probable loan losses
Balance at the beginning of the period	$ 27,905	25,649
Losses charged to allowance	(2,715)	(2,735)
Recoveries credited to allowance	583	699
Net (losses) recoveries charged to allowance	(2,132)	(2,036)
Provision charged to operations	(5,082)	2,240
Balance at the end of the period	20,691	25,853
Commercial Real Estate:Other Construction and Land Development
Allowance for probable loan losses
Balance at the beginning of the period	11,675	13,889
Losses charged to allowance	(1)
Recoveries credited to allowance	2	1
Net (losses) recoveries charged to allowance	1	1
Provision charged to operations	122	(101)
Balance at the end of the period	11,798	13,789
Commercial Real Estate: Farmland and Commercial
Allowance for probable loan losses
Balance at the beginning of the period	16,663	16,731
Recoveries credited to allowance	18	58
Net (losses) recoveries charged to allowance	18	58
Provision charged to operations	8,122	(68)
Balance at the end of the period	24,803	16,721
Commercial Real Estate: Multifamily
Allowance for probable loan losses
Balance at the beginning of the period	1,109	806
Provision charged to operations	(162)	12
Balance at the end of the period	947	818
Residential First Lien
Allowance for probable loan losses
Balance at the beginning of the period	2,950	2,455
Losses charged to allowance	(14)	(31)
Recoveries credited to allowance	1	5
Net (losses) recoveries charged to allowance	(13)	(26)
Provision charged to operations	69	(38)
Balance at the end of the period	3,006	2,391
Residential Junior Lien
Allowance for probable loan losses
Balance at the beginning of the period	6,103	3,716
Losses charged to allowance	(30)	(105)
Recoveries credited to allowance	66	24
Net (losses) recoveries charged to allowance	36	(81)
Provision charged to operations	(1,450)	(449)
Balance at the end of the period	4,689	3,186
Consumer
Allowance for probable loan losses
Balance at the beginning of the period	440	531
Losses charged to allowance	(117)	(121)
Recoveries credited to allowance	12	16
Net (losses) recoveries charged to allowance	(105)	(105)
Provision charged to operations	102	78
Balance at the end of the period	437	504
Foreign
Allowance for probable loan losses
Balance at the beginning of the period	842	884
Recoveries credited to allowance		14
Net (losses) recoveries charged to allowance		14
Provision charged to operations	(59)	26
Balance at the end of the period	$ 783	$ 924