Loans - Schedule of Recorded Investment in Criticized Loans by Class and Credit Quality Indicator (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	$ 7,567,720	$ 5,416,750
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,822,900	1,834,400
Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	348,700	208,500
Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	240,200	147,900
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	119,400	125,600
Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	708,300	482,000
Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,487,400	1,027,400
Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	158,200	170,200
Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	5,176,800	3,514,000
Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	784,700	752,400
Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	175,100	148,100
Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,034,400	970,300
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,215,400	797,900
Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	136,200	132,900
Other Assets Especial Mentioned
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	155,100	163,600
Other Assets Especial Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	78,000	85,300
Other Assets Especial Mentioned | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,200	13,400
Other Assets Especial Mentioned | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,300	400
Other Assets Especial Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,400	1,600
Other Assets Especial Mentioned | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	7,900	15,400
Other Assets Especial Mentioned | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,900	5,000
Other Assets Especial Mentioned | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	4,300	3,800
Other Assets Especial Mentioned | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	94,100	109,500
Other Assets Especial Mentioned | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	800	800
Other Assets Especial Mentioned | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	400	700
Other Assets Especial Mentioned | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,200	1,500
Other Assets Especial Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	54,700	46,400
Other Assets Especial Mentioned | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	5,100	6,200
Substandard
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	217,200	172,200
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	96,400	85,300
Substandard | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	16,400	6,200
Substandard | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,700	1,600
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,600	6,300
Substandard | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	21,700	14,100
Substandard | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	12,500	12,500
Substandard | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	19,100	17,800
Substandard | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	149,700	129,700
Substandard | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,200	1,500
Substandard | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	700	1,000
Substandard | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,900	2,500
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	56,300	29,300
Substandard | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	8,300	10,700
Doubtful
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	28,200	36,400
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	10,300	10,800
Doubtful | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0	1,400
Doubtful | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	500	700
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,500	700
Doubtful | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	4,000	2,800
Doubtful | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,900	800
Doubtful | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0	0
Doubtful | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	16,200	14,400
Doubtful | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	300	100
Doubtful | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	200	300
Doubtful | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	500	400
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	11,100	21,200
Doubtful | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	400	400
Total Criticized Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	400,500	372,200
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	184,700	181,400
Total Criticized Loans | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	19,600	21,000
Total Criticized Loans | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	4,500	2,700
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	9,500	8,600
Total Criticized Loans | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	33,600	32,300
Total Criticized Loans | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	18,300	18,300
Total Criticized Loans | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	23,400	21,600
Total Criticized Loans | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	260,000	253,600
Total Criticized Loans | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,300	2,400
Total Criticized Loans | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,300	2,000
Total Criticized Loans | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	4,600	4,400
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	122,100	96,900
Total Criticized Loans | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	$ 13,800	$ 17,300