Loans and Leases - Credit Quality Indicators (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment	$ 5,211,856,000	$ 4,386,836,000	$ 4,251,933,000
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		3,454,955,000
Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		67,798,000
Grade 3: Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		71,285,000
Grade 4: Doubtful [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment	0
Internally assigned grades [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		3,594,038,000
Performing
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		788,650,000
Nonperforming
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		4,148,000
Performing and nonperforming [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		792,798,000
Commercial Financial And Agricultural [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment	894,314,000	947,029,000	959,153,000
Commercial Financial And Agricultural [Member] | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		911,848,000
Commercial Financial And Agricultural [Member] | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		18,843,000
Commercial Financial And Agricultural [Member] | Grade 3: Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		16,338,000
Commercial Financial And Agricultural [Member] | Internally assigned grades [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		947,029,000
Real-Estate Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment	2,369,691,000	2,040,441,000
Real-Estate Commercial [Member] | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		1,974,561,000
Real-Estate Commercial [Member] | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		24,199,000
Real-Estate Commercial [Member] | Grade 3: Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		41,681,000
Real-Estate Commercial [Member] | Internally assigned grades [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		2,040,441,000
Real-Estate Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment	233,590,000	232,595,000
Real-Estate Construction [Member] | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		201,424,000
Real-Estate Construction [Member] | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		20,987,000
Real-Estate Construction [Member] | Grade 3: Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		10,184,000
Real-Estate Construction [Member] | Internally assigned grades [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		232,595,000
Real Estate Residential Secured For Business Purpose [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment	378,239,000	373,973,000	360,721,000
Real Estate Residential Secured For Business Purpose [Member] | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		367,122,000
Real Estate Residential Secured For Business Purpose [Member] | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		3,769,000
Real Estate Residential Secured For Business Purpose [Member] | Grade 3: Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		3,082,000
Real Estate Residential Secured For Business Purpose [Member] | Internally assigned grades [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		373,973,000
Real Estate-Residential Secured For Personal Purpose [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment	474,688,000	439,059,000
Real Estate-Residential Secured For Personal Purpose [Member] | Performing
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		437,021,000
Real Estate-Residential Secured For Personal Purpose [Member] | Nonperforming
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		2,038,000
Real Estate-Residential Secured For Personal Purpose [Member] | Performing and nonperforming [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		439,059,000
Real Estate Home Equity Secured For Personal Purpose [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment	172,448,000	174,435,000
Real Estate Home Equity Secured For Personal Purpose [Member] | Performing
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		172,954,000
Real Estate Home Equity Secured For Personal Purpose [Member] | Nonperforming
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		1,481,000
Real Estate Home Equity Secured For Personal Purpose [Member] | Performing and nonperforming [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		174,435,000
Loans to Individuals [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment	27,771,000	29,883,000	30,715,000
Loans to Individuals [Member] | Performing
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		29,809,000
Loans to Individuals [Member] | Nonperforming
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		74,000
Loans to Individuals [Member] | Performing and nonperforming [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		29,883,000
Lease Financings [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment	$ 159,535,000	149,421,000	$ 140,807,000
Lease Financings [Member] | Performing
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		148,866,000
Lease Financings [Member] | Nonperforming
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		555,000
Lease Financings [Member] | Performing and nonperforming [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and leases held for investment		$ 149,421,000