Allowance for Probable Loan Losses (By Loan Class) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Allowance for probable loan losses
Balance at the beginning of the period	$ 64,186	$ 61,784	$ 64,661	$ 66,988
Losses charged to allowance	(2,406)	(7,535)	(5,398)	(27,110)
Recoveries credited to allowance	2,334	687	3,151	5,924
Net (losses) recoveries charged to allowance	(72)	(6,848)	(2,247)	(21,186)
Provision charged to operations	805	7,097	2,505	16,231
Balance at the end of the period	64,919	62,033	$ 64,919	62,033
Period of time for determining the historical charge-off experience			3 years
Watch - List Impaired
Allowance for probable loan losses
Losses charged to allowance		(16,800)		(16,800)
Commercial
Allowance for probable loan losses
Balance at the beginning of the period	25,853	25,783	$ 25,649	21,431
Losses charged to allowance	(2,264)	(5,396)	(4,999)	(24,467)
Recoveries credited to allowance	2,154	513	2,853	5,656
Net (losses) recoveries charged to allowance	(110)	(4,883)	(2,146)	(18,811)
Provision charged to operations	(603)	5,082	1,637	23,362
Balance at the end of the period	25,140	25,982	25,140	25,982
Commercial Real Estate: Other Construction and Land Development
Allowance for probable loan losses
Balance at the beginning of the period	13,789	10,870	13,889	13,920
Losses charged to allowance		(2)		(2)
Recoveries credited to allowance	2	3	3	7
Net (losses) recoveries charged to allowance	2	1	3	5
Provision charged to operations	(542)	238	(643)	(2,816)
Balance at the end of the period	13,249	11,109	13,249	11,109
Commercial Real Estate: Farmland & Commercial
Allowance for probable loan losses
Balance at the beginning of the period	16,721	16,974	16,731	19,769
Losses charged to allowance	(40)	(1,843)	(40)	(1,890)
Recoveries credited to allowance	89	75	147	86
Net (losses) recoveries charged to allowance	49	(1,768)	107	(1,804)
Provision charged to operations	702	1,736	634	(1,023)
Balance at the end of the period	17,472	16,942	17,472	16,942
Commercial Real Estate: Multifamily
Allowance for probable loan losses
Balance at the beginning of the period	818	910	806	1,248
Losses charged to allowance				(180)
Net (losses) recoveries charged to allowance				(180)
Provision charged to operations	179	(55)	191	(213)
Balance at the end of the period	997	855	997	855
Residential First Lien
Allowance for probable loan losses
Balance at the beginning of the period	2,391	2,369	2,455	3,509
Losses charged to allowance	(30)	(23)	(61)	(30)
Recoveries credited to allowance	2	4	7	7
Net (losses) recoveries charged to allowance	(28)	(19)	(54)	(23)
Provision charged to operations	(49)	(73)	(87)	(1,209)
Balance at the end of the period	2,314	2,277	2,314	2,277
Residential Junior Lien
Allowance for probable loan losses
Balance at the beginning of the period	3,186	3,420	3,716	5,321
Losses charged to allowance	(33)	(155)	(138)	(324)
Recoveries credited to allowance	73	76	97	114
Net (losses) recoveries charged to allowance	40	(79)	(41)	(210)
Provision charged to operations	1,131	59	682	(1,711)
Balance at the end of the period	4,357	3,400	4,357	3,400
Consumer
Allowance for probable loan losses
Balance at the beginning of the period	504	530	531	638
Losses charged to allowance	(39)	(116)	(160)	(217)
Recoveries credited to allowance	13	5	29	42
Net (losses) recoveries charged to allowance	(26)	(111)	(131)	(175)
Provision charged to operations	10	113	88	69
Balance at the end of the period	488	532	488	532
Foreign
Allowance for probable loan losses
Balance at the beginning of the period	924	928	884	1,152
Recoveries credited to allowance	1	11	15	12
Net (losses) recoveries charged to allowance	1	11	15	12
Provision charged to operations	(23)	(3)	3	(228)
Balance at the end of the period	$ 902	$ 936	$ 902	$ 936