Allowance for Probable Loan Losses (By Loan Class) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Allowance for probable loan losses
Balance at the beginning of the period	$ 64,919	$ 62,033	$ 64,661	$ 66,988
Losses charged to allowance	(4,743)	(4,188)	(10,141)	(31,298)
Recoveries credited to allowance	1,085	7,472	4,236	13,396
Net (losses) recoveries charged to allowance	(3,658)	3,284	(5,905)	(17,902)
Provision charged (credited) to operations	6,591	(1,347)	9,096	14,884
Balance at the end of the period	67,852	63,970	$ 67,852	63,970
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	25,140	25,982	$ 25,649	21,431
Losses charged to allowance	(4,249)	(3,485)	(9,248)	(27,952)
Recoveries credited to allowance	631	791	3,484	6,447
Net (losses) recoveries charged to allowance	(3,618)	(2,694)	(5,764)	(21,505)
Provision charged (credited) to operations	5,473	2,602	7,110	25,964
Balance at the end of the period	26,995	25,890	26,995	25,890
Commercial Real Estate: Other Construction and Land Development
Allowance for probable loan losses
Balance at the beginning of the period	13,249	11,109	13,889	13,920
Losses charged to allowance	(213)	(14)	(213)	(16)
Recoveries credited to allowance	17	6,073	20	6,080
Net (losses) recoveries charged to allowance	(196)	6,059	(193)	6,064
Provision charged (credited) to operations	(1,168)	(4,659)	(1,811)	(7,475)
Balance at the end of the period	11,885	12,509	11,885	12,509
Commercial Real Estate: Farmland & Commercial
Allowance for probable loan losses
Balance at the beginning of the period	17,472	16,942	16,731	19,769
Losses charged to allowance		(497)	(40)	(2,387)
Recoveries credited to allowance	341	512	488	598
Net (losses) recoveries charged to allowance	341	15	448	(1,789)
Provision charged (credited) to operations	371	291	1,005	(732)
Balance at the end of the period	18,184	17,248	18,184	17,248
Commercial Real Estate: Multifamily
Allowance for probable loan losses
Balance at the beginning of the period	997	855	806	1,248
Losses charged to allowance				(180)
Net (losses) recoveries charged to allowance				(180)
Provision charged (credited) to operations		(65)	191	(278)
Balance at the end of the period	997	790	997	790
Residential First Lien
Allowance for probable loan losses
Balance at the beginning of the period	2,314	2,277	2,455	3,509
Losses charged to allowance	(16)	(27)	(77)	(57)
Recoveries credited to allowance		11	7	18
Net (losses) recoveries charged to allowance	(16)	(16)	(70)	(39)
Provision charged (credited) to operations	817	126	730	(1,083)
Balance at the end of the period	3,115	2,387	3,115	2,387
Residential Junior Lien
Allowance for probable loan losses
Balance at the beginning of the period	4,357	3,400	3,716	5,321
Losses charged to allowance	(202)		(340)	(324)
Recoveries credited to allowance	85	74	182	188
Net (losses) recoveries charged to allowance	(117)	74	(158)	(136)
Provision charged (credited) to operations	995	221	1,677	(1,490)
Balance at the end of the period	5,235	3,695	5,235	3,695
Consumer
Allowance for probable loan losses
Balance at the beginning of the period	488	532	531	638
Losses charged to allowance	(63)	(124)	(223)	(341)
Recoveries credited to allowance	6	11	35	53
Net (losses) recoveries charged to allowance	(57)	(113)	(188)	(288)
Provision charged (credited) to operations	82	98	170	167
Balance at the end of the period	513	517	513	517
Foreign
Allowance for probable loan losses
Balance at the beginning of the period	902	936	884	1,152
Losses charged to allowance		(41)		(41)
Recoveries credited to allowance	5		20	12
Net (losses) recoveries charged to allowance	5	(41)	20	(29)
Provision charged (credited) to operations	21	39	24	(189)
Balance at the end of the period	$ 928	$ 934	$ 928	$ 934