Allowance for Probable Loan Losses (Portfolio Credit Quality By Loan Class) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) item	Dec. 31, 2016 USD ($)
Loan portfolio by credit quality indicator
Portfolio, total	$ 6,202,264	$ 5,964,688
Number of additional relationships reclassified from Pass to Watch List - Substandard | item	3
Pass
Loan portfolio by credit quality indicator
Portfolio, total	$ 5,775,814	5,544,614
Special Review
Loan portfolio by credit quality indicator
Portfolio, total	11,763	101,679
Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	39,928	60,993
Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	336,590	208,997
Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	38,169	48,405
Commercial
Loan portfolio by credit quality indicator
Portfolio, total	1,029,041	909,667
Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	855,329	720,350
Commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	52	90,746
Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	3,128	1,121
Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	150,339	75,038
Commercial | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	20,193	22,412
Commercial Real Estate: Other Construction and Land Development
Loan portfolio by credit quality indicator
Portfolio, total	1,704,804	1,716,875
Commercial Real Estate: Other Construction and Land Development | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,638,432	1,648,633
Commercial Real Estate: Other Construction and Land Development | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	912	1,986
Commercial Real Estate: Other Construction and Land Development | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	63,032	61,480
Commercial Real Estate: Other Construction and Land Development | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	2,428	4,776
Commercial Real Estate: Farmland & Commercial
Loan portfolio by credit quality indicator
Portfolio, total	1,975,842	1,943,070
Commercial Real Estate: Farmland & Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,799,806	1,792,542
Commercial Real Estate: Farmland & Commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	10,608	7,983
Commercial Real Estate: Farmland & Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	36,800	59,872
Commercial Real Estate: Farmland & Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	122,662	71,863
Commercial Real Estate: Farmland & Commercial | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	5,966	10,810
Commercial Real Estate: Multifamily
Loan portfolio by credit quality indicator
Portfolio, total	167,377	140,466
Commercial Real Estate: Multifamily | Pass
Loan portfolio by credit quality indicator
Portfolio, total	166,881	139,914
Commercial Real Estate: Multifamily | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	496	552
Residential First Lien
Loan portfolio by credit quality indicator
Portfolio, total	427,855	421,904
Residential First Lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	420,606	413,638
Residential First Lien | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	41	814
Residential First Lien | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	557	616
Residential First Lien | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	6,651	6,836
Residential Junior Lien
Loan portfolio by credit quality indicator
Portfolio, total	680,516	610,318
Residential Junior Lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	679,604	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	762	978
Consumer
Loan portfolio by credit quality indicator
Portfolio, total	53,959	55,168
Consumer | Pass
Loan portfolio by credit quality indicator
Portfolio, total	52,641	53,873
Consumer | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	1,318	1,295
Foreign
Loan portfolio by credit quality indicator
Portfolio, total	162,870	167,220
Foreign | Pass
Loan portfolio by credit quality indicator
Portfolio, total	162,515	166,474
Foreign | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	$ 355	$ 746