Loans and Allowance for Loan Losses - Age Analysis of Past Due Loans, Excluding Loans Acquired, Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans	$ 11,723,171	$ 10,779,685
Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	36,687	46,075
Current	8,393,701	5,659,534
Total Loans	8,430,388	5,705,609
90 Days Past Due & Accruing	224	343
Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	21,917	18,159
90 Days or More Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	14,770	27,916
Consumer | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	6,699	9,686
Current	398,771	455,830
Total Loans	405,470	465,516
90 Days Past Due & Accruing	213	342
Consumer | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	5,297	5,716
Consumer | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,402	3,970
Consumer | Credit cards | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,539	1,379
Current	202,634	184,043
Total Loans	204,173	185,422
90 Days Past Due & Accruing	209	332
Consumer | Credit cards | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,033	707
Consumer | Credit cards | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	506	672
Consumer | Other consumer | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	5,160	8,307
Current	196,137	271,787
Total Loans	201,297	280,094
90 Days Past Due & Accruing	4	10
Consumer | Other consumer | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	4,264	5,009
Consumer | Other consumer | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	896	3,298
Real Estate | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	18,889	26,571
Current	5,947,564	4,213,041
Total Loans	5,966,453	4,239,612
90 Days Past Due & Accruing	0	1
Real Estate | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	11,681	10,870
Real Estate | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	7,208	15,701
Real Estate | Construction | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	841	1,621
Current	1,299,882	612,534
Total Loans	1,300,723	614,155
90 Days Past Due & Accruing	0	0
Real Estate | Construction | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	533	411
Real Estate | Construction | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	308	1,210
Real Estate | Single family residential | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	11,896	14,531
Current	1,428,547	1,080,102
Total Loans	1,440,443	1,094,633
90 Days Past Due & Accruing	0	1
Real Estate | Single family residential | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	7,769	8,071
Real Estate | Single family residential | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	4,127	6,460
Real Estate | Other commercial | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	6,152	10,419
Current	3,219,135	2,520,405
Total Loans	3,225,287	2,530,824
90 Days Past Due & Accruing	0	0
Real Estate | Other commercial | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	3,379	2,388
Real Estate | Other commercial | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,773	8,031
Commercial | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	11,099	9,818
Current	1,928,324	963,701
Total Loans	1,939,423	973,519
90 Days Past Due & Accruing	11	0
Commercial | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	4,939	1,573
Commercial | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	6,160	8,245
Commercial | Commercial | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	9,577	7,648
Current	1,765,332	817,569
Total Loans	1,774,909	825,217
90 Days Past Due & Accruing	11	0
Commercial | Commercial | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	4,472	1,523
Commercial | Commercial | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	5,105	6,125
Commercial | Agricultural | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,522	2,170
Current	162,992	146,132
Total Loans	164,514	148,302
90 Days Past Due & Accruing	0	0
Commercial | Agricultural | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	467	50
Commercial | Agricultural | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,055	2,120
Other | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Current	119,042	26,962
Total Loans	119,042	26,962
90 Days Past Due & Accruing	0	$ 0
Other | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0
Other | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 0