Loans and Allowance for Loan Losses - Age Analysis of Past Due Loans, Excluding Loans Acquired, Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	$ 11,858,195	$ 10,779,685
Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	38,907	46,075
Current	8,084,367	5,659,534
Total Loans	8,123,274	5,705,609
90 Days Past Due & Accruing	281	343
Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	19,582	18,159
90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	19,325	27,916
Consumer | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6,556	9,686
Current	435,162	455,830
Total Loans	441,718	465,516
90 Days Past Due & Accruing	189	342
Consumer | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,118	5,716
Consumer | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,438	3,970
Consumer | Credit cards | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,294	1,379
Current	180,843	184,043
Total Loans	182,137	185,422
90 Days Past Due & Accruing	188	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	777	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	517	672
Consumer | Other consumer | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,262	8,307
Current	254,319	271,787
Total Loans	259,581	280,094
90 Days Past Due & Accruing	1	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	3,341	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	1,921	3,298
Real estate | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	19,529	26,571
Current	5,689,538	4,213,041
Total Loans	5,709,067	4,239,612
90 Days Past Due & Accruing	92	1
Real estate | Gross 30-89 Days Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	10,768	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	8,761	15,701
Real estate | Construction | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,430	1,621
Current	1,228,458	612,534
Total Loans	1,229,888	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	1,033	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	397	1,210
Real estate | Single family residential | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	9,392	14,531
Current	1,392,599	1,080,102
Total Loans	1,401,991	1,094,633
90 Days Past Due & Accruing	92	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	5,180	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	4,212	6,460
Real estate | Other commercial | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	8,707	10,419
Current	3,068,481	2,520,405
Total Loans	3,077,188	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	4,555	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	4,152	8,031
Commercial | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	12,822	9,818
Current	1,814,298	963,701
Total Loans	1,827,120	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	4,696	1,573
Commercial | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	8,126	8,245
Commercial | Commercial | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	10,441	7,648
Current	1,597,901	817,569
Total Loans	1,608,342	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	4,568	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	5,873	6,125
Commercial | Agricultural | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,381	2,170
Current	216,397	146,132
Total Loans	218,778	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	128	50
Commercial | Agricultural | 90 Days or More Past Due | Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,253	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	145,369	26,962
Total Loans	145,369	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