Allowance for Probable Loan Losses (Portfolio Credit Quality By Loan Class) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Loan portfolio by credit quality indicator
Portfolio, total	$ 6,009,503	$ 5,964,688
Pass
Loan portfolio by credit quality indicator
Portfolio, total	5,607,839	5,544,614
Special Review
Loan portfolio by credit quality indicator
Portfolio, total	43,923	101,679
Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	97,971	60,993
Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	216,757	208,997
Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	43,013	48,405
Commercial
Loan portfolio by credit quality indicator
Portfolio, total	864,193	909,667
Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	688,694	720,350
Commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	35,310	90,746
Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	38,253	1,121
Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	80,600	75,038
Commercial | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	21,336	22,412
Commercial Real Estate: Other Construction and Land Development
Loan portfolio by credit quality indicator
Portfolio, total	1,778,282	1,716,875
Commercial Real Estate: Other Construction and Land Development | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,711,225	1,648,633
Commercial Real Estate: Other Construction and Land Development | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	923	1,986
Commercial Real Estate: Other Construction and Land Development | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	62,547	61,480
Commercial Real Estate: Other Construction and Land Development | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	3,587	4,776
Commercial Real Estate: Farmland & Commercial
Loan portfolio by credit quality indicator
Portfolio, total	1,957,672	1,943,070
Commercial Real Estate: Farmland & Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,809,418	1,792,542
Commercial Real Estate: Farmland & Commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	7,496	7,983
Commercial Real Estate: Farmland & Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	59,718	59,872
Commercial Real Estate: Farmland & Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	73,043	71,863
Commercial Real Estate: Farmland & Commercial | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	7,997	10,810
Commercial Real Estate: Multifamily
Loan portfolio by credit quality indicator
Portfolio, total	145,098	140,466
Commercial Real Estate: Multifamily | Pass
Loan portfolio by credit quality indicator
Portfolio, total	144,566	139,914
Commercial Real Estate: Multifamily | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	532	552
Residential First Lien
Loan portfolio by credit quality indicator
Portfolio, total	418,534	421,904
Residential First Lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	411,333	413,638
Residential First Lien | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	44	814
Residential First Lien | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	567	616
Residential First Lien | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	6,590	6,836
Residential Junior Lien
Loan portfolio by credit quality indicator
Portfolio, total	628,481	610,318
Residential Junior Lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	627,363	609,190
Residential Junior Lien | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	150	150
Residential Junior Lien | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	968	978
Consumer
Loan portfolio by credit quality indicator
Portfolio, total	53,614	55,168
Consumer | Pass
Loan portfolio by credit quality indicator
Portfolio, total	52,368	53,873
Consumer | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	1,246	1,295
Foreign
Loan portfolio by credit quality indicator
Portfolio, total	163,629	167,220
Foreign | Pass
Loan portfolio by credit quality indicator
Portfolio, total	162,872	166,474
Foreign | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	$ 757	$ 746