Loans and Allowance for Loan Losses - Details on Allowance for Loan Losses and Recorded Investment by Loan Classification and Impairment Evaluation Method (Details) - USD ($)		Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment		$ 81,435,000	$ 78,934,000
Individually evaluated for impairment		5,518,000	3,964,000
Total allowance for loan losses		87,194,000	83,575,000	$ 70,204,000	$ 67,240,000
Collectively evaluated for impairment		18,055,200,000	17,599,074,000
Individually evaluated for impairment		96,004,000	83,146,000
Loans		18,174,906,000	17,707,549,000
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		23,702,000	25,329,000
Commercial real estate - mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment		28,646,000	26,222,000
Individually evaluated for impairment		1,521,000	724,000
Total allowance for loan losses		30,167,000	26,946,000	22,688,000	21,188,000
Collectively evaluated for impairment		7,372,359,000	7,124,117,000
Individually evaluated for impairment		35,213,000	28,838,000
Loans		7,419,146,000	7,164,954,000
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]	0	0
Loans acquired with deteriorated credit quality		11,574,000	11,999,000
Consumer real estate - mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment		5,988,000	6,227,000
Individually evaluated for impairment		2,381,000	1,443,000
Total allowance for loan losses		8,369,000	7,670,000	5,100,000	5,031,000
Collectively evaluated for impairment		2,844,051,000	2,808,142,000
Individually evaluated for impairment		35,099,000	27,111,000
Loans		2,887,628,000	2,844,447,000
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]	0	0
Loans acquired with deteriorated credit quality		8,478,000	9,194,000
Construction and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment		10,860,000	11,100,000
Individually evaluated for impairment		55,000	28,000
Total allowance for loan losses		10,915,000	11,128,000	10,116,000	8,962,000
Collectively evaluated for impairment		2,093,358,000	2,066,613,000
Individually evaluated for impairment		868,000	2,367,000
Loans		2,097,570,000	2,072,455,000
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	0
Loans acquired with deteriorated credit quality		3,344,000	3,475,000
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment		31,238,000	30,290,000
Individually evaluated for impairment		1,461,000	1,441,000
Total allowance for loan losses		32,699,000	31,731,000	26,648,000	24,863,000
Collectively evaluated for impairment		5,394,721,000	5,246,913,000
Individually evaluated for impairment		24,493,000	23,847,000
Loans		5,419,520,000	5,271,421,000
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	0
Loans acquired with deteriorated credit quality		306,000	661,000
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment		4,703,000	5,095,000
Individually evaluated for impairment		100,000	328,000
Total allowance for loan losses		4,803,000	5,423,000	5,476,000	5,874,000
Collectively evaluated for impairment		350,711,000	353,289,000
Individually evaluated for impairment		331,000	983,000
Loans		351,042,000	354,272,000
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		241,000	677,000	$ 176,000	$ 1,322,000
Unallocated | Loans acquired with deteriorated credit quality(1)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total allowance for loan losses	[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 only in the event of subsequent credit deterioration.