Allowance for Probable Loan Losses (By Loan Class) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Allowance for probable loan losses
Balance at the beginning of the period	$ 62,853	$ 64,919	$ 67,687	$ 64,661
Losses charged to allowance	(7,088)	(4,743)	(12,424)	(10,141)
Recoveries credited to allowance	609	1,085	2,179	4,236
Net losses (recoveries) charged to allowance	(6,479)	(3,658)	(10,245)	(5,905)
Provision charged to operations	4,280	6,591	3,212	9,096
Balance at the end of the period	60,654	67,852	$ 60,654	67,852
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	16,596	25,140	$ 27,905	25,649
Losses charged to allowance	(6,798)	(4,249)	(11,798)	(9,248)
Recoveries credited to allowance	489	631	1,520	3,484
Net losses (recoveries) charged to allowance	(6,309)	(3,618)	(10,278)	(5,764)
Provision charged to operations	3,039	5,473	(4,301)	7,110
Balance at the end of the period	13,326	26,995	13,326	26,995
Commercial Real Estate:Other Construction and Land Development
Allowance for probable loan losses
Balance at the beginning of the period	14,703	13,249	11,675	13,889
Losses charged to allowance		(213)	(1)	(213)
Recoveries credited to allowance	4	17	8	20
Net losses (recoveries) charged to allowance	4	(196)	7	(193)
Provision charged to operations	397	(1,168)	3,422	(1,811)
Balance at the end of the period	15,104	11,885	15,104	11,885
Commercial Real Estate: Farmland and Commercial
Allowance for probable loan losses
Balance at the beginning of the period	18,296	17,472	16,663	16,731
Losses charged to allowance			(70)	(40)
Recoveries credited to allowance	19	341	229	488
Net losses (recoveries) charged to allowance	19	341	159	448
Provision charged to operations	1,050	371	2,543	1,005
Balance at the end of the period	19,365	18,184	19,365	18,184
Commercial Real Estate: Multifamily
Allowance for probable loan losses
Balance at the beginning of the period	1,574	997	1,109	806
Provision charged to operations	(138)		327	191
Balance at the end of the period	1,436	997	1,436	997
Residential First Lien
Allowance for probable loan losses
Balance at the beginning of the period	3,591	2,314	2,950	2,455
Losses charged to allowance	(74)	(16)	(118)	(77)
Recoveries credited to allowance	23		25	7
Net losses (recoveries) charged to allowance	(51)	(16)	(93)	(70)
Provision charged to operations	(354)	817	329	730
Balance at the end of the period	3,186	3,115	3,186	3,115
Residential Junior Lien
Allowance for probable loan losses
Balance at the beginning of the period	6,793	4,357	6,103	3,716
Losses charged to allowance	(133)	(202)	(172)	(340)
Recoveries credited to allowance	53	85	348	182
Net losses (recoveries) charged to allowance	(80)	(117)	176	(158)
Provision charged to operations	194	995	628	1,677
Balance at the end of the period	6,907	5,235	6,907	5,235
Consumer
Allowance for probable loan losses
Balance at the beginning of the period	450	488	440	531
Losses charged to allowance	(80)	(63)	(262)	(223)
Recoveries credited to allowance	12	6	39	35
Net losses (recoveries) charged to allowance	(68)	(57)	(223)	(188)
Provision charged to operations	57	82	222	170
Balance at the end of the period	439	513	439	513
Foreign
Allowance for probable loan losses
Balance at the beginning of the period	850	902	842	884
Losses charged to allowance	(3)		(3)
Recoveries credited to allowance	9	5	10	20
Net losses (recoveries) charged to allowance	6	5	7	20
Provision charged to operations	35	21	42	24
Balance at the end of the period	$ 891	$ 928	$ 891	$ 928