Loans and Allowance for Credit Losses - Risk Grade and Category of Loan (Details) - USD ($) $ in Thousands		Jun. 30, 2019		Dec. 31, 2018
Financing Receivable Impaired [Line Items]
Loans		$ 10,587,375		$ 10,370,313
Agriculture and Agriculture Real Estate (Includes Farmland) [Member]
Financing Receivable Impaired [Line Items]
Loans		736,470		729,501
Real Estate [Member] | Construction, Land Development and Other Land Loans [Member]
Financing Receivable Impaired [Line Items]
Loans		1,739,308		1,622,289
Real Estate [Member] | 1-4 Family Residential (Includes Home Equity) [Member]
Financing Receivable Impaired [Line Items]
Loans	[1]	2,713,278	[2]	2,706,909	[3]
Real Estate [Member] | Commercial Real Estate (Includes Multi-Family Residential) [Member]
Financing Receivable Impaired [Line Items]
Loans		3,551,668		3,538,557
Commercial and Industrial [Member]
Financing Receivable Impaired [Line Items]
Loans		1,525,628		1,483,571
Consumer and Other [Member]
Financing Receivable Impaired [Line Items]
Loans		321,023		289,486
Grade 1 [Member]
Financing Receivable Impaired [Line Items]
Loans		104,975		112,839
Grade 1 [Member] | Agriculture and Agriculture Real Estate (Includes Farmland) [Member]
Financing Receivable Impaired [Line Items]
Loans		14,322		15,725
Grade 1 [Member] | Real Estate [Member] | Construction, Land Development and Other Land Loans [Member]
Financing Receivable Impaired [Line Items]
Loans		0		0
Grade 1 [Member] | Real Estate [Member] | 1-4 Family Residential (Includes Home Equity) [Member]
Financing Receivable Impaired [Line Items]
Loans		0	[2]	0	[3]
Grade 1 [Member] | Real Estate [Member] | Commercial Real Estate (Includes Multi-Family Residential) [Member]
Financing Receivable Impaired [Line Items]
Loans		0		0
Grade 1 [Member] | Commercial and Industrial [Member]
Financing Receivable Impaired [Line Items]
Loans		55,562		59,979
Grade 1 [Member] | Consumer and Other [Member]
Financing Receivable Impaired [Line Items]
Loans		35,091		37,135
Grade 2 [Member]
Financing Receivable Impaired [Line Items]
Loans		110,993		115,491
Grade 2 [Member] | Agriculture and Agriculture Real Estate (Includes Farmland) [Member]
Financing Receivable Impaired [Line Items]
Loans		3,877		3,974
Grade 2 [Member] | Real Estate [Member] | Construction, Land Development and Other Land Loans [Member]
Financing Receivable Impaired [Line Items]
Loans		1,215		1,040
Grade 2 [Member] | Real Estate [Member] | 1-4 Family Residential (Includes Home Equity) [Member]
Financing Receivable Impaired [Line Items]
Loans		15,430	[2]	21,465	[3]
Grade 2 [Member] | Real Estate [Member] | Commercial Real Estate (Includes Multi-Family Residential) [Member]
Financing Receivable Impaired [Line Items]
Loans		21,640		22,207
Grade 2 [Member] | Commercial and Industrial [Member]
Financing Receivable Impaired [Line Items]
Loans		12,312		11,003
Grade 2 [Member] | Consumer and Other [Member]
Financing Receivable Impaired [Line Items]
Loans		56,519		55,802
Grade 3 [Member]
Financing Receivable Impaired [Line Items]
Loans		9,031,777		8,974,366
Grade 3 [Member] | Agriculture and Agriculture Real Estate (Includes Farmland) [Member]
Financing Receivable Impaired [Line Items]
Loans		647,171		636,674
Grade 3 [Member] | Real Estate [Member] | Construction, Land Development and Other Land Loans [Member]
Financing Receivable Impaired [Line Items]
Loans		1,587,922		1,509,532
Grade 3 [Member] | Real Estate [Member] | 1-4 Family Residential (Includes Home Equity) [Member]
Financing Receivable Impaired [Line Items]
Loans		2,600,773	[2]	2,598,600	[3]
Grade 3 [Member] | Real Estate [Member] | Commercial Real Estate (Includes Multi-Family Residential) [Member]
Financing Receivable Impaired [Line Items]
Loans		2,892,964		2,974,474
Grade 3 [Member] | Commercial and Industrial [Member]
Financing Receivable Impaired [Line Items]
Loans		1,102,365		1,083,328
Grade 3 [Member] | Consumer and Other [Member]
Financing Receivable Impaired [Line Items]
Loans		200,582		171,758
Grade 4 [Member]
Financing Receivable Impaired [Line Items]
Loans		1,114,794		972,809
Grade 4 [Member] | Agriculture and Agriculture Real Estate (Includes Farmland) [Member]
Financing Receivable Impaired [Line Items]
Loans		64,025		66,650
Grade 4 [Member] | Real Estate [Member] | Construction, Land Development and Other Land Loans [Member]
Financing Receivable Impaired [Line Items]
Loans		136,083		99,087
Grade 4 [Member] | Real Estate [Member] | 1-4 Family Residential (Includes Home Equity) [Member]
Financing Receivable Impaired [Line Items]
Loans		67,445	[2]	61,430	[3]
Grade 4 [Member] | Real Estate [Member] | Commercial Real Estate (Includes Multi-Family Residential) [Member]
Financing Receivable Impaired [Line Items]
Loans		557,501		481,735
Grade 4 [Member] | Commercial and Industrial [Member]
Financing Receivable Impaired [Line Items]
Loans		265,415		243,743
Grade 4 [Member] | Consumer and Other [Member]
Financing Receivable Impaired [Line Items]
Loans		24,325		20,164
Grade 5 [Member]
Financing Receivable Impaired [Line Items]
Loans		138,336		120,781
Grade 5 [Member] | Agriculture and Agriculture Real Estate (Includes Farmland) [Member]
Financing Receivable Impaired [Line Items]
Loans		5,091		5,578
Grade 5 [Member] | Real Estate [Member] | Construction, Land Development and Other Land Loans [Member]
Financing Receivable Impaired [Line Items]
Loans		5,684		3,673
Grade 5 [Member] | Real Estate [Member] | 1-4 Family Residential (Includes Home Equity) [Member]
Financing Receivable Impaired [Line Items]
Loans		6,486	[2]	12,522	[3]
Grade 5 [Member] | Real Estate [Member] | Commercial Real Estate (Includes Multi-Family Residential) [Member]
Financing Receivable Impaired [Line Items]
Loans		56,772		37,942
Grade 5 [Member] | Commercial and Industrial [Member]
Financing Receivable Impaired [Line Items]
Loans		61,476		58,088
Grade 5 [Member] | Consumer and Other [Member]
Financing Receivable Impaired [Line Items]
Loans		2,827		2,978
Grade 6 [Member]
Financing Receivable Impaired [Line Items]
Loans		41,266		52,383
Grade 6 [Member] | Agriculture and Agriculture Real Estate (Includes Farmland) [Member]
Financing Receivable Impaired [Line Items]
Loans		1,029		282
Grade 6 [Member] | Real Estate [Member] | Construction, Land Development and Other Land Loans [Member]
Financing Receivable Impaired [Line Items]
Loans		7,315		7,081
Grade 6 [Member] | Real Estate [Member] | 1-4 Family Residential (Includes Home Equity) [Member]
Financing Receivable Impaired [Line Items]
Loans		3,750	[2]	4,332	[3]
Grade 6 [Member] | Real Estate [Member] | Commercial Real Estate (Includes Multi-Family Residential) [Member]
Financing Receivable Impaired [Line Items]
Loans		15,719		16,006
Grade 6 [Member] | Commercial and Industrial [Member]
Financing Receivable Impaired [Line Items]
Loans		11,814		23,081
Grade 6 [Member] | Consumer and Other [Member]
Financing Receivable Impaired [Line Items]
Loans		1,639		1,601
Grade 7 [Member]
Financing Receivable Impaired [Line Items]
Loans		36,981		12,975
Grade 7 [Member] | Agriculture and Agriculture Real Estate (Includes Farmland) [Member]
Financing Receivable Impaired [Line Items]
Loans		326		256
Grade 7 [Member] | Real Estate [Member] | Construction, Land Development and Other Land Loans [Member]
Financing Receivable Impaired [Line Items]
Loans		659		1,384
Grade 7 [Member] | Real Estate [Member] | 1-4 Family Residential (Includes Home Equity) [Member]
Financing Receivable Impaired [Line Items]
Loans		15,278	[2]	4,395	[3]
Grade 7 [Member] | Real Estate [Member] | Commercial Real Estate (Includes Multi-Family Residential) [Member]
Financing Receivable Impaired [Line Items]
Loans		4,135		2,727
Grade 7 [Member] | Commercial and Industrial [Member]
Financing Receivable Impaired [Line Items]
Loans		16,543		4,165
Grade 7 [Member] | Consumer and Other [Member]
Financing Receivable Impaired [Line Items]
Loans		40		48
Grade 8 [Member]
Financing Receivable Impaired [Line Items]
Loans		308		81
Grade 8 [Member] | Agriculture and Agriculture Real Estate (Includes Farmland) [Member]
Financing Receivable Impaired [Line Items]
Loans		290		0
Grade 8 [Member] | Real Estate [Member] | Construction, Land Development and Other Land Loans [Member]
Financing Receivable Impaired [Line Items]
Loans		0		0
Grade 8 [Member] | Real Estate [Member] | 1-4 Family Residential (Includes Home Equity) [Member]
Financing Receivable Impaired [Line Items]
Loans		0	[2]	48	[3]
Grade 8 [Member] | Real Estate [Member] | Commercial Real Estate (Includes Multi-Family Residential) [Member]
Financing Receivable Impaired [Line Items]
Loans		0		0
Grade 8 [Member] | Commercial and Industrial [Member]
Financing Receivable Impaired [Line Items]
Loans		18		33
Grade 8 [Member] | Consumer and Other [Member]
Financing Receivable Impaired [Line Items]
Loans		0		0
Grade 9 [Member]
Financing Receivable Impaired [Line Items]
Loans		0		0
Grade 9 [Member] | Agriculture and Agriculture Real Estate (Includes Farmland) [Member]
Financing Receivable Impaired [Line Items]
Loans		0		0
Grade 9 [Member] | Real Estate [Member] | Construction, Land Development and Other Land Loans [Member]
Financing Receivable Impaired [Line Items]
Loans		0		0
Grade 9 [Member] | Real Estate [Member] | 1-4 Family Residential (Includes Home Equity) [Member]
Financing Receivable Impaired [Line Items]
Loans		0	[2]	0	[3]
Grade 9 [Member] | Real Estate [Member] | Commercial Real Estate (Includes Multi-Family Residential) [Member]
Financing Receivable Impaired [Line Items]
Loans		0		0
Grade 9 [Member] | Commercial and Industrial [Member]
Financing Receivable Impaired [Line Items]
Loans		0		0
Grade 9 [Member] | Consumer and Other [Member]
Financing Receivable Impaired [Line Items]
Loans		0		0
PCI Loans [Member]
Financing Receivable Impaired [Line Items]
Loans		7,945		8,588
PCI Loans [Member] | Agriculture and Agriculture Real Estate (Includes Farmland) [Member]
Financing Receivable Impaired [Line Items]
Loans		339		362
PCI Loans [Member] | Real Estate [Member] | Construction, Land Development and Other Land Loans [Member]
Financing Receivable Impaired [Line Items]
Loans		430		492
PCI Loans [Member] | Real Estate [Member] | 1-4 Family Residential (Includes Home Equity) [Member]
Financing Receivable Impaired [Line Items]
Loans		4,116	[2]	4,117	[3]
PCI Loans [Member] | Real Estate [Member] | Commercial Real Estate (Includes Multi-Family Residential) [Member]
Financing Receivable Impaired [Line Items]
Loans		2,937		3,466
PCI Loans [Member] | Commercial and Industrial [Member]
Financing Receivable Impaired [Line Items]
Loans		123		151
PCI Loans [Member] | Consumer and Other [Member]
Financing Receivable Impaired [Line Items]
Loans		$ 0		$ 0

[1] Includes $20.3 million and $29.4 million of residential mortgage loans held for sale at June 30, 2019 and December 31, 2018, respectively.

[2] Includes $20.3 million of residential mortgage loans held for sale at June 30, 2019.

[3] Includes $29.4 million of residential mortgage loans held for sale at December 31, 2018.