Loans and Allowance for Loan Losses - Age Analysis of Past Due Loans, Excluding Loans Acquired, Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	$ 11,740,737	$ 11,723,171
Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	48,036	36,687
Current	8,636,514	8,393,701
Total Loans	8,684,550	8,430,388
90 Days Past Due & Accruing	281	224
Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	26,369	21,917
90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	21,667	14,770
Consumer | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,801	6,699
Current	390,407	398,771
Total Loans	395,208	405,470
90 Days Past Due & Accruing	274	213
Consumer | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,751	5,297
Consumer | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,050	1,402
Consumer | Credit cards | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,294	1,539
Current	180,255	202,634
Total Loans	181,549	204,173
90 Days Past Due & Accruing	222	209
Consumer | Credit cards | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	733	1,033
Consumer | Credit cards | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	561	506
Consumer | Other consumer | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,507	5,160
Current	210,152	196,137
Total Loans	213,659	201,297
90 Days Past Due & Accruing	52	4
Consumer | Other consumer | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,018	4,264
Consumer | Other consumer | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	489	896
Real estate | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	26,623	18,889
Current	6,136,055	5,947,564
Total Loans	6,162,678	5,966,453
90 Days Past Due & Accruing	7	0
Real estate | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	15,571	11,681
Real estate | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	11,052	7,208
Real estate | Construction | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,870	841
Current	1,374,292	1,299,882
Total Loans	1,376,162	1,300,723
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	961	533
Real estate | Construction | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	909	308
Real estate | Single family residential | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	15,717	11,896
Current	1,415,690	1,428,547
Total Loans	1,431,407	1,440,443
90 Days Past Due & Accruing	7	0
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	10,226	7,769
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	5,491	4,127
Real estate | Other commercial | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	9,036	6,152
Current	3,346,073	3,219,135
Total Loans	3,355,109	3,225,287
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	4,384	3,379
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	4,652	2,773
Commercial | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	16,612	11,099
Current	1,932,026	1,928,324
Total Loans	1,948,638	1,939,423
90 Days Past Due & Accruing	0	11
Commercial | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	7,047	4,939
Commercial | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	9,565	6,160
Commercial | Commercial | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	15,456	9,577
Current	1,785,966	1,765,332
Total Loans	1,801,422	1,774,909
90 Days Past Due & Accruing	0	11
Commercial | Commercial | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6,845	4,472
Commercial | Commercial | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	8,611	5,105
Commercial | Agricultural | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,156	1,522
Current	146,060	162,992
Total Loans	147,216	164,514
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	202	467
Commercial | Agricultural | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	954	1,055
Other | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
Current	178,026	119,042
Total Loans	178,026	119,042
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