Loans and Allowance for Loan Losses - Loans by Credit Risk Rating, Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 11,858,195	$ 10,779,685
Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	11,644,398	10,438,375
Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	68,510	165,736
Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	144,691	175,064
Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	596	376
Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	134
Other
Financing Receivable, Recorded Investment [Line Items]
Loans		26,962
Other | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans		26,962
Other | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans		0
Other | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans		0
Other | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans		0
Other | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans		0
Loans, Excluding Acquired Loans
Financing Receivable, Recorded Investment [Line Items]
Loans	8,123,274	5,705,609
Loans, Excluding Acquired Loans | Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans	441,718	465,516
Loans, Excluding Acquired Loans | Consumer | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	437,477	460,080
Loans, Excluding Acquired Loans | Consumer | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans, Excluding Acquired Loans | Consumer | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	4,241	5,436
Loans, Excluding Acquired Loans | Consumer | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans, Excluding Acquired Loans | Consumer | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans, Excluding Acquired Loans | Consumer | Credit cards
Financing Receivable, Recorded Investment [Line Items]
Loans	182,137	185,422
Loans, Excluding Acquired Loans | Consumer | Credit cards | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	181,620	184,920
Loans, Excluding Acquired Loans | Consumer | Credit cards | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans, Excluding Acquired Loans | Consumer | Credit cards | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	517	502
Loans, Excluding Acquired Loans | Consumer | Credit cards | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans, Excluding Acquired Loans | Consumer | Credit cards | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans, Excluding Acquired Loans | Consumer | Other consumer
Financing Receivable, Recorded Investment [Line Items]
Loans	259,581	280,094
Loans, Excluding Acquired Loans | Consumer | Other consumer | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	255,857	275,160
Loans, Excluding Acquired Loans | Consumer | Other consumer | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans, Excluding Acquired Loans | Consumer | Other consumer | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	3,724	4,934
Loans, Excluding Acquired Loans | Consumer | Other consumer | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans, Excluding Acquired Loans | Consumer | Other consumer | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans, Excluding Acquired Loans | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	5,709,067	4,239,612
Loans, Excluding Acquired Loans | Real estate | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	5,655,650	4,150,321
Loans, Excluding Acquired Loans | Real estate | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	12,433	29,330
Loans, Excluding Acquired Loans | Real estate | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	40,747	59,658
Loans, Excluding Acquired Loans | Real estate | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	237	303
Loans, Excluding Acquired Loans | Real estate | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans, Excluding Acquired Loans | Real estate | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	1,229,888	614,155
Loans, Excluding Acquired Loans | Real estate | Construction | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	1,225,481	603,126
Loans, Excluding Acquired Loans | Real estate | Construction | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	2,286	5,795
Loans, Excluding Acquired Loans | Real estate | Construction | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	2,121	5,218
Loans, Excluding Acquired Loans | Real estate | Construction | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	0	16
Loans, Excluding Acquired Loans | Real estate | Construction | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans, Excluding Acquired Loans | Real estate | Single family residential
Financing Receivable, Recorded Investment [Line Items]
Loans	1,401,991	1,094,633
Loans, Excluding Acquired Loans | Real estate | Single family residential | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	1,382,655	1,066,902
Loans, Excluding Acquired Loans | Real estate | Single family residential | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	843	3,954
Loans, Excluding Acquired Loans | Real estate | Single family residential | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	18,256	23,490
Loans, Excluding Acquired Loans | Real estate | Single family residential | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	237	287
Loans, Excluding Acquired Loans | Real estate | Single family residential | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans, Excluding Acquired Loans | Real estate | Other commercial
Financing Receivable, Recorded Investment [Line Items]
Loans	3,077,188	2,530,824
Loans, Excluding Acquired Loans | Real estate | Other commercial | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	3,047,514	2,480,293
Loans, Excluding Acquired Loans | Real estate | Other commercial | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	9,304	19,581
Loans, Excluding Acquired Loans | Real estate | Other commercial | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	20,370	30,950
Loans, Excluding Acquired Loans | Real estate | Other commercial | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans, Excluding Acquired Loans | Real estate | Other commercial | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans, Excluding Acquired Loans | Commercial
Financing Receivable, Recorded Investment [Line Items]
Loans	1,827,120	973,519
Loans, Excluding Acquired Loans | Commercial | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	1,785,099	882,442
Loans, Excluding Acquired Loans | Commercial | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	8,405	74,278
Loans, Excluding Acquired Loans | Commercial | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	33,616	16,592
Loans, Excluding Acquired Loans | Commercial | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	0	73
Loans, Excluding Acquired Loans | Commercial | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	134
Loans, Excluding Acquired Loans | Commercial | Commercial
Financing Receivable, Recorded Investment [Line Items]
Loans	1,608,342	825,217
Loans, Excluding Acquired Loans | Commercial | Commercial | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	1,569,357	736,377
Loans, Excluding Acquired Loans | Commercial | Commercial | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	8,250	74,254
Loans, Excluding Acquired Loans | Commercial | Commercial | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	30,735	14,402
Loans, Excluding Acquired Loans | Commercial | Commercial | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	0	50
Loans, Excluding Acquired Loans | Commercial | Commercial | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	134
Loans, Excluding Acquired Loans | Commercial | Agricultural
Financing Receivable, Recorded Investment [Line Items]
Loans	218,778	148,302
Loans, Excluding Acquired Loans | Commercial | Agricultural | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	215,742	146,065
Loans, Excluding Acquired Loans | Commercial | Agricultural | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	155	24
Loans, Excluding Acquired Loans | Commercial | Agricultural | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	2,881	2,190
Loans, Excluding Acquired Loans | Commercial | Agricultural | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	0	23
Loans, Excluding Acquired Loans | Commercial | Agricultural | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans, Excluding Acquired Loans | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	145,369	26,962
Loans, Excluding Acquired Loans | Other | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	145,369
Loans, Excluding Acquired Loans | Other | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	0
Loans, Excluding Acquired Loans | Other | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	0
Loans, Excluding Acquired Loans | Other | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	0
Loans, Excluding Acquired Loans | Other | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0
Loans Acquired
Financing Receivable, Recorded Investment [Line Items]
Loans	3,734,921	5,074,076
Loans Acquired | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	3,620,803	4,918,570
Loans Acquired | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	47,672	62,128
Loans Acquired | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	66,087	93,378
Loans Acquired | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	359	0
Loans Acquired | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans Acquired | Consumer | Other consumer
Financing Receivable, Recorded Investment [Line Items]
Loans	21,278	51,467
Loans Acquired | Consumer | Other consumer | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	20,813	50,625
Loans Acquired | Consumer | Other consumer | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	9	21
Loans Acquired | Consumer | Other consumer | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	456	821
Loans Acquired | Consumer | Other consumer | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans Acquired | Consumer | Other consumer | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans Acquired | Real estate
Financing Receivable, Recorded Investment [Line Items]
Loans	3,114,950	3,818,037
Loans Acquired | Real estate | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	3,037,255	3,712,847
Loans Acquired | Real estate | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	32,769	48,206
Loans Acquired | Real estate | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	44,567	56,984
Loans Acquired | Real estate | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	359	0
Loans Acquired | Real estate | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans Acquired | Real estate | Other consumer
Financing Receivable, Recorded Investment [Line Items]
Loans	2,040,189	2,387,777
Loans Acquired | Real estate | Other consumer | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	2,006,371
Loans Acquired | Real estate | Other consumer | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	12,283
Loans Acquired | Real estate | Other consumer | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	21,535
Loans Acquired | Real estate | Other consumer | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	0
Loans Acquired | Real estate | Other consumer | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0
Loans Acquired | Real estate | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	467,895	637,032
Loans Acquired | Real estate | Construction | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	440,881	604,796
Loans Acquired | Real estate | Construction | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	18,011	30,524
Loans Acquired | Real estate | Construction | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	9,003	1,712
Loans Acquired | Real estate | Construction | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans Acquired | Real estate | Construction | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans Acquired | Real estate | Single family residential
Financing Receivable, Recorded Investment [Line Items]
Loans	606,866	793,228
Loans Acquired | Real estate | Single family residential | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	590,003	770,954
Loans Acquired | Real estate | Single family residential | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	2,475	2,618
Loans Acquired | Real estate | Single family residential | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	14,029	19,656
Loans Acquired | Real estate | Single family residential | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	359	0
Loans Acquired | Real estate | Single family residential | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans Acquired | Real estate | Other commercial
Financing Receivable, Recorded Investment [Line Items]
Loans	2,040,189	2,387,777
Loans Acquired | Real estate | Other commercial | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans		2,337,097
Loans Acquired | Real estate | Other commercial | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans		15,064
Loans Acquired | Real estate | Other commercial | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans		35,616
Loans Acquired | Real estate | Other commercial | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans		0
Loans Acquired | Real estate | Other commercial | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans		0
Loans Acquired | Commercial
Financing Receivable, Recorded Investment [Line Items]
Loans	598,693	1,062,163
Loans Acquired | Commercial | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	562,735	1,012,689
Loans Acquired | Commercial | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	14,894	13,901
Loans Acquired | Commercial | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	21,064	35,573
Loans Acquired | Commercial | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans Acquired | Commercial | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans Acquired | Commercial | Commercial
Financing Receivable, Recorded Investment [Line Items]
Loans	596,641	995,587
Loans Acquired | Commercial | Commercial | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	560,791	946,322
Loans Acquired | Commercial | Commercial | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	14,894	13,901
Loans Acquired | Commercial | Commercial | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	20,956	35,364
Loans Acquired | Commercial | Commercial | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans Acquired | Commercial | Commercial | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans Acquired | Commercial | Agricultural
Financing Receivable, Recorded Investment [Line Items]
Loans	2,052	66,576
Loans Acquired | Commercial | Agricultural | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans	1,944	66,367
Loans Acquired | Commercial | Agricultural | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans Acquired | Commercial | Agricultural | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans	108	209
Loans Acquired | Commercial | Agricultural | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans Acquired | Commercial | Agricultural | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans	0	0
Loans Acquired | Other
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 0	142,409
Loans Acquired | Other | Risk Rate 1-4
Financing Receivable, Recorded Investment [Line Items]
Loans		142,409
Loans Acquired | Other | Risk Rate 5
Financing Receivable, Recorded Investment [Line Items]
Loans		0
Loans Acquired | Other | Risk Rate 6
Financing Receivable, Recorded Investment [Line Items]
Loans		0
Loans Acquired | Other | Risk Rate 7
Financing Receivable, Recorded Investment [Line Items]
Loans		0
Loans Acquired | Other | Risk Rate 8
Financing Receivable, Recorded Investment [Line Items]
Loans		$ 0