Loans and Allowance for Loan Losses - Age Analysis of Past Due Loans, Excluding Loans Acquired, Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	$ 11,365,895	$ 10,779,685
Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	40,516	46,075
Current	7,092,945	5,659,534
Total Loans	7,133,461	5,705,609
90 Days Past Due & Accruing	303	343
Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	16,419	18,159
90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	24,097	27,916
Consumer | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6,414	9,686
Current	451,268	455,830
Total Loans	457,682	465,516
90 Days Past Due & Accruing	182	342
Consumer | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,668	5,716
Consumer | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,746	3,970
Consumer | Credit cards | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,025	1,379
Current	179,327	184,043
Total Loans	180,352	185,422
90 Days Past Due & Accruing	145	332
Consumer | Credit cards | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	642	707
Consumer | Credit cards | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	383	672
Consumer | Other consumer | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,389	8,307
Current	271,941	271,787
Total Loans	277,330	280,094
90 Days Past Due & Accruing	37	10
Consumer | Other consumer | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,026	5,009
Consumer | Other consumer | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,363	3,298
Real estate | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	23,363	26,571
Current	5,075,564	4,213,041
Total Loans	5,098,927	4,239,612
90 Days Past Due & Accruing	121	1
Real estate | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	8,564	10,870
Real estate | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	14,799	15,701
Real estate | Construction | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,336	1,621
Current	966,384	612,534
Total Loans	967,720	614,155
90 Days Past Due & Accruing	0	0
Real estate | Construction | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	515	411
Real estate | Construction | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	821	1,210
Real estate | Single family residential | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	14,085	14,531
Current	1,300,702	1,080,102
Total Loans	1,314,787	1,094,633
90 Days Past Due & Accruing	121	1
Real estate | Single family residential | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6,187	8,071
Real estate | Single family residential | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	7,898	6,460
Real estate | Other commercial | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	7,942	10,419
Current	2,808,478	2,520,405
Total Loans	2,816,420	2,530,824
90 Days Past Due & Accruing	0	0
Real estate | Other commercial | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,862	2,388
Real estate | Other commercial | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6,080	8,031
Commercial | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	10,739	9,818
Current	1,414,177	963,701
Total Loans	1,424,916	973,519
90 Days Past Due & Accruing	0	0
Commercial | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,187	1,573
Commercial | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6,552	8,245
Commercial | Commercial | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	8,543	7,648
Current	1,229,367	817,569
Total Loans	1,237,910	825,217
90 Days Past Due & Accruing	0	0
Commercial | Commercial | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,995	1,523
Commercial | Commercial | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,548	6,125
Commercial | Agricultural | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,196	2,170
Current	184,810	146,132
Total Loans	187,006	148,302
90 Days Past Due & Accruing	0	0
Commercial | Agricultural | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	192	50
Commercial | Agricultural | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,004	2,120
Other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans		26,962
Other | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Current	151,936	26,962
Total Loans	151,936	26,962
90 Days Past Due & Accruing	0	$ 0
Other | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
Other | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 0