Loans and Allowance for Loan Losses - Details on Allowance for Loan Losses and Recorded Investment by Loan Classification and Impairment Evaluation Method (Details) - USD ($)		Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment		$ 76,774,000		$ 63,147,000
Individually evaluated for impairment		2,378,000		2,198,000
Total allowance for loan losses		79,985,000	$ 75,670,000	67,240,000	$ 65,159,000	$ 61,944,000	$ 58,980,000
Collectively evaluated for impairment		17,346,542,000		15,531,482,000
Individually evaluated for impairment		83,861,000		58,979,000
Loans		17,464,009,000		15,633,116,000
Loans acquired with deteriorated credit quality (1)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total allowance for loan losses	[1]	11,000		573,000
Loans acquired with deteriorated credit quality		33,606,000		42,655,000
Commercial real estate - mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment		26,047,000		20,753,000
Individually evaluated for impairment		461,000		95,000
Total allowance for loan losses		26,517,000	24,848,000	21,188,000	20,790,000	16,002,000	13,655,000
Collectively evaluated for impairment		7,164,591,000		6,630,593,000
Individually evaluated for impairment		31,739,000		18,214,000
Loans		7,215,119,000		6,669,610,000
Commercial real estate - mortgage | Loans acquired with deteriorated credit quality (1)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total allowance for loan losses	[1]	9,000		340,000
Loans acquired with deteriorated credit quality		18,789,000		20,803,000
Consumer real estate - mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment		6,235,000		4,460,000
Individually evaluated for impairment		490,000		410,000
Total allowance for loan losses		6,727,000	5,853,000	5,031,000	5,303,000	7,835,000	6,564,000
Collectively evaluated for impairment		2,783,196,000		2,534,996,000
Individually evaluated for impairment		22,128,000		12,172,000
Loans		2,815,160,000		2,561,214,000
Consumer real estate - mortgage | Loans acquired with deteriorated credit quality (1)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total allowance for loan losses	[1]	2,000		161,000
Loans acquired with deteriorated credit quality		9,836,000		14,046,000
Construction and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment		10,967,000		8,879,000
Individually evaluated for impairment		15,000		66,000
Total allowance for loan losses		10,982,000	10,984,000	8,962,000	7,523,000	5,126,000	3,624,000
Collectively evaluated for impairment		2,051,413,000		1,896,553,000
Individually evaluated for impairment		3,447,000		5,167,000
Loans		2,059,009,000		1,908,288,000
Construction and land development | Loans acquired with deteriorated credit quality (1)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total allowance for loan losses	[1]	0		17,000
Loans acquired with deteriorated credit quality		4,149,000		6,568,000
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment		28,859,000		23,181,000
Individually evaluated for impairment		1,265,000		1,627,000
Total allowance for loan losses		30,124,000	28,338,000	24,863,000	23,407,000	24,235,000	24,743,000
Collectively evaluated for impairment		4,979,829,000		4,116,677,000
Individually evaluated for impairment		25,586,000		23,426,000
Loans		5,006,247,000		4,141,341,000
Commercial and industrial | Loans acquired with deteriorated credit quality (1)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total allowance for loan losses	[1]	0		55,000
Loans acquired with deteriorated credit quality		832,000		1,238,000
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment		4,666,000		5,874,000
Individually evaluated for impairment		147,000		0
Total allowance for loan losses		4,813,000	5,172,000	5,874,000	6,919,000	7,549,000	9,520,000
Collectively evaluated for impairment		367,513,000		352,663,000
Individually evaluated for impairment		961,000		0
Loans		368,474,000		352,663,000
Consumer and other | Loans acquired with deteriorated credit quality (1)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total allowance for loan losses	[1]	0		0
Loans acquired with deteriorated credit quality		0		0
Unallocated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment
Individually evaluated for impairment
Total allowance for loan losses		822,000	$ 475,000	1,322,000	$ 1,217,000	$ 1,197,000	$ 874,000
Unallocated | Loans acquired with deteriorated credit quality (1)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total allowance for loan losses	[1]

[1] (1) Loans acquired with deteriorated credit quality are recorded at fair value at the time of acquisition. An allowance for loan losses is recorded resulting from subsequent credit deterioration.