Allowance for Probable Loan Losses (By Loan Class) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Allowance for probable loan losses
Balance at the beginning of the period	$ 67,154	$ 64,186	$ 67,687	$ 64,661
Losses charged to allowance	(2,458)	(2,406)	(5,335)	(5,398)
Recoveries credited to allowance	887	2,334	1,569	3,151
Net (losses) recoveries charged to allowance	(1,571)	(72)	(3,766)	(2,247)
Provision charged to operations	(2,730)	805	(1,068)	2,505
Balance at the end of the period	62,853	64,919	$ 62,853	64,919
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	20,691	25,853	$ 27,905	25,649
Losses charged to allowance	(2,284)	(2,264)	(4,999)	(4,999)
Recoveries credited to allowance	447	2,154	1,030	2,853
Net (losses) recoveries charged to allowance	(1,837)	(110)	(3,969)	(2,146)
Provision charged to operations	(2,258)	(603)	(7,340)	1,637
Balance at the end of the period	16,596	25,140	16,596	25,140
Commercial Real Estate:Other Construction and Land Development
Allowance for probable loan losses
Balance at the beginning of the period	11,798	13,789	11,675	13,889
Losses charged to allowance			(1)
Recoveries credited to allowance	2	2	4	3
Net (losses) recoveries charged to allowance	2	2	3	3
Provision charged to operations	2,903	(542)	3,025	(643)
Balance at the end of the period	14,703	13,249	14,703	13,249
Commercial Real Estate: Farmland and Commercial
Allowance for probable loan losses
Balance at the beginning of the period	24,803	16,721	16,663	16,731
Losses charged to allowance	(70)	(40)	(70)	(40)
Recoveries credited to allowance	192	89	210	147
Net (losses) recoveries charged to allowance	122	49	140	107
Provision charged to operations	(6,629)	702	1,493	634
Balance at the end of the period	18,296	17,472	18,296	17,472
Commercial Real Estate: Multifamily
Allowance for probable loan losses
Balance at the beginning of the period	947	818	1,109	806
Provision charged to operations	627	179	465	191
Balance at the end of the period	1,574	997	1,574	997
Residential First Lien
Allowance for probable loan losses
Balance at the beginning of the period	3,006	2,391	2,950	2,455
Losses charged to allowance	(30)	(30)	(44)	(61)
Recoveries credited to allowance	1	2	2	7
Net (losses) recoveries charged to allowance	(29)	(28)	(42)	(54)
Provision charged to operations	614	(49)	683	(87)
Balance at the end of the period	3,591	2,314	3,591	2,314
Residential Junior Lien
Allowance for probable loan losses
Balance at the beginning of the period	4,689	3,186	6,103	3,716
Losses charged to allowance	(9)	(33)	(39)	(138)
Recoveries credited to allowance	229	73	295	97
Net (losses) recoveries charged to allowance	220	40	256	(41)
Provision charged to operations	1,884	1,131	434	682
Balance at the end of the period	6,793	4,357	6,793	4,357
Consumer
Allowance for probable loan losses
Balance at the beginning of the period	437	504	440	531
Losses charged to allowance	(65)	(39)	(182)	(160)
Recoveries credited to allowance	15	13	27	29
Net (losses) recoveries charged to allowance	(50)	(26)	(155)	(131)
Provision charged to operations	63	10	165	88
Balance at the end of the period	450	488	450	488
Foreign
Allowance for probable loan losses
Balance at the beginning of the period	783	924	842	884
Recoveries credited to allowance	1	1	1	15
Net (losses) recoveries charged to allowance	1	1	1	15
Provision charged to operations	66	(23)	7	3
Balance at the end of the period	$ 850	$ 902	$ 850	$ 902