Loans - Schedule of Recorded Investment in Criticized Loans by Class and Credit Quality Indicator (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	$ 8,470.4	$ 7,567.7
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,235.4	2,822.9
Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	321.6	348.7
Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	242.8	240.2
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	274.3	119.4
Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	838.7	708.3
Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,542.0	1,487.4
Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	217.4	158.2
Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	5,833.5	5,176.8
Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	787.8	784.7
Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	200.6	175.1
Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,070.2	1,034.4
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,310.3	1,215.4
Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	254.8	136.2
Other Assets Especial Mentioned
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	178.8	155.1
Other Assets Especial Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	102.5	78.0
Other Assets Especial Mentioned | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	5.0	3.2
Other Assets Especial Mentioned | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2.8	2.3
Other Assets Especial Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1.7	2.4
Other Assets Especial Mentioned | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	9.5	7.9
Other Assets Especial Mentioned | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3.0	3.9
Other Assets Especial Mentioned | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	9.0	4.3
Other Assets Especial Mentioned | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	124.0	94.1
Other Assets Especial Mentioned | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.7	0.8
Other Assets Especial Mentioned | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.3	0.4
Other Assets Especial Mentioned | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1.0	1.2
Other Assets Especial Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	39.4	54.7
Other Assets Especial Mentioned | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	14.4	5.1
Substandard
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	201.6	217.2
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	87.4	96.4
Substandard | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	7.0	16.4
Substandard | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2.0	1.7
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3.9	3.6
Substandard | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	12.9	21.7
Substandard | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	10.8	12.5
Substandard | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	24.0	19.1
Substandard | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	135.1	149.7
Substandard | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2.1	2.2
Substandard | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.8	0.7
Substandard | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2.9	2.9
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	45.8	56.3
Substandard | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	17.8	8.3
Doubtful
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	20.9	28.2
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2.9	10.3
Doubtful | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3.3	0.0
Doubtful | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.4	0.5
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.0	3.5
Doubtful | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3.7	4.0
Doubtful | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.7	1.9
Doubtful | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.1	0.0
Doubtful | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	7.4	16.2
Doubtful | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.1	0.3
Doubtful | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.1	0.2
Doubtful | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.2	0.5
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	11.8	11.1
Doubtful | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1.5	0.4
Total Criticized Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	401.3	400.5
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	192.8	184.7
Total Criticized Loans | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	15.3	19.6
Total Criticized Loans | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	5.2	4.5
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	5.6	9.5
Total Criticized Loans | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	26.1	33.6
Total Criticized Loans | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	14.5	18.3
Total Criticized Loans | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	33.1	23.4
Total Criticized Loans | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	266.5	260.0
Total Criticized Loans | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2.9	3.3
Total Criticized Loans | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1.2	1.3
Total Criticized Loans | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	4.1	4.6
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	97.0	122.1
Total Criticized Loans | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	$ 33.7	$ 13.8