Allowance for Probable Loan Losses (Portfolio Credit Quality By Loan Class) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($) item	Dec. 31, 2018 USD ($)
Loan portfolio by credit quality indicator
Portfolio, total	$ 6,895,142	$ 6,561,289
Number of additional relationships reclassified from Pass to Watch List - Substandard | item	2
Pass
Loan portfolio by credit quality indicator
Portfolio, total	$ 6,524,425	6,126,933
Special Review
Loan portfolio by credit quality indicator
Portfolio, total	7,994	64,135
Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	73,574	92,976
Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	269,160	253,345
Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	19,989	23,900
Commercial
Loan portfolio by credit quality indicator
Portfolio, total	1,295,736	1,128,969
Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,228,895	998,625
Commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	19	441
Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	957	44,544
Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	58,453	76,180
Commercial | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	7,412	9,179
Commercial Real Estate:Other Construction and Land Development
Loan portfolio by credit quality indicator
Portfolio, total	2,044,410	1,886,231
Commercial Real Estate:Other Construction and Land Development | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,948,752	1,817,098
Commercial Real Estate:Other Construction and Land Development | Special Review
Loan portfolio by credit quality indicator
Portfolio, total		1,648
Commercial Real Estate:Other Construction and Land Development | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	37,566	9,055
Commercial Real Estate:Other Construction and Land Development | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	56,298	56,338
Commercial Real Estate:Other Construction and Land Development | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	1,794	2,092
Commercial Real Estate: Farmland and Commercial
Loan portfolio by credit quality indicator
Portfolio, total	1,967,111	1,949,898
Commercial Real Estate: Farmland and Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,771,180	1,726,711
Commercial Real Estate: Farmland and Commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	7,975	62,046
Commercial Real Estate: Farmland and Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	34,067	38,373
Commercial Real Estate: Farmland and Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	152,149	119,259
Commercial Real Estate: Farmland and Commercial | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	1,740	3,509
Commercial Real Estate: Multifamily
Loan portfolio by credit quality indicator
Portfolio, total	242,089	226,257
Commercial Real Estate: Multifamily | Pass
Loan portfolio by credit quality indicator
Portfolio, total	240,535	224,823
Commercial Real Estate: Multifamily | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	905	927
Commercial Real Estate: Multifamily | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	649	507
Residential First Lien
Loan portfolio by credit quality indicator
Portfolio, total	438,472	445,800
Residential First Lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	430,825	438,773
Residential First Lien | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	143	142
Residential First Lien | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	1,218	641
Residential First Lien | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	6,286	6,244
Residential Junior Lien
Loan portfolio by credit quality indicator
Portfolio, total	713,292	727,301
Residential Junior Lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	711,747	725,538
Residential Junior Lien | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	841	862
Residential Junior Lien | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	704	901
Consumer
Loan portfolio by credit quality indicator
Portfolio, total	47,072	46,316
Consumer | Pass
Loan portfolio by credit quality indicator
Portfolio, total	45,941	45,141
Consumer | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	1,131	1,175
Foreign
Loan portfolio by credit quality indicator
Portfolio, total	146,960	150,517
Foreign | Pass
Loan portfolio by credit quality indicator
Portfolio, total	146,550	150,224
Foreign | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	137
Foreign | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	$ 273	$ 293