Allowance for Probable Loan Losses (By Loan Class) (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Allowance for probable loan losses
Balance at the beginning of the period	$ 58,203		$ 61,384	$ 62,853	$ 61,384	$ 67,687
Losses charged to allowance	(4,293)			(7,088)	(19,172)	(12,424)
Recoveries credited to allowance	672			609	2,285	2,179
Net losses (recoveries) charged to allowance	(3,621)			(6,479)	(16,887)	(10,245)
Provision charged to operations	5,278			4,280	15,363	3,212
Balance at the end of the period	59,860	$ 58,203		60,654	59,860	60,654
Watch - List Impaired
Allowance for probable loan losses
Losses charged to allowance		9,500	9,500
Commercial
Allowance for probable loan losses
Balance at the beginning of the period	11,780		12,596	16,596	12,596	27,905
Losses charged to allowance	(3,231)			(6,798)	(11,011)	(11,798)
Recoveries credited to allowance	585			489	1,631	1,520
Net losses (recoveries) charged to allowance	(2,646)			(6,309)	(9,380)	(10,278)
Provision charged to operations	1,837			3,039	7,755	(4,301)
Balance at the end of the period	10,971	11,780		13,326	10,971	13,326
Commercial Real Estate:Other Construction and Land Development
Allowance for probable loan losses
Balance at the beginning of the period	15,273		15,123	14,703	15,123	11,675
Losses charged to allowance						(1)
Recoveries credited to allowance	4			4	80	8
Net losses (recoveries) charged to allowance	4			4	80	7
Provision charged to operations	1,573			397	1,647	3,422
Balance at the end of the period	16,850	15,273		15,104	16,850	15,104
Commercial Real Estate: Farmland and Commercial
Allowance for probable loan losses
Balance at the beginning of the period	17,029		19,353	18,296	19,353	16,663
Losses charged to allowance	(468)				(7,347)	(70)
Recoveries credited to allowance	10			19	308	229
Net losses (recoveries) charged to allowance	(458)			19	(7,039)	159
Provision charged to operations	230			1,050	4,487	2,543
Balance at the end of the period	16,801	17,029		19,365	16,801	19,365
Commercial Real Estate: Multifamily
Allowance for probable loan losses
Balance at the beginning of the period	1,849		1,808	1,574	1,808	1,109
Provision charged to operations	350			(138)	391	327
Balance at the end of the period	2,199	1,849		1,436	2,199	1,436
Residential First Lien
Allowance for probable loan losses
Balance at the beginning of the period	3,627		3,467	3,591	3,467	2,950
Losses charged to allowance	(164)			(74)	(166)	(118)
Recoveries credited to allowance	3			23	14	25
Net losses (recoveries) charged to allowance	(161)			(51)	(152)	(93)
Provision charged to operations	263			(354)	414	329
Balance at the end of the period	3,729	3,627		3,186	3,729	3,186
Residential Junior Lien
Allowance for probable loan losses
Balance at the beginning of the period	7,312		7,719	6,793	7,719	6,103
Losses charged to allowance	(214)			(133)	(314)	(172)
Recoveries credited to allowance	65			53	222	348
Net losses (recoveries) charged to allowance	(149)			(80)	(92)	176
Provision charged to operations	852			194	388	628
Balance at the end of the period	8,015	7,312		6,907	8,015	6,907
Consumer
Allowance for probable loan losses
Balance at the beginning of the period	488		447	450	447	440
Losses charged to allowance	(216)			(80)	(334)	(262)
Recoveries credited to allowance	5			12	30	39
Net losses (recoveries) charged to allowance	(211)			(68)	(304)	(223)
Provision charged to operations	199			57	333	222
Balance at the end of the period	476	488		439	476	439
Foreign
Allowance for probable loan losses
Balance at the beginning of the period	845		$ 871	850	871	842
Losses charged to allowance				(3)		(3)
Recoveries credited to allowance				9		10
Net losses (recoveries) charged to allowance				6		7
Provision charged to operations	(26)			35	(52)	42
Balance at the end of the period	$ 819	$ 845		$ 891	$ 819	$ 891