Loans - Schedule of Recorded Investment in Criticized Loans by Class and Credit Quality Indicator (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	$ 7,614.5	$ 7,567.7
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	2,821.9	2,822.9
Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	364.6	348.7
Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	230.6	240.2
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	140.8	119.4
Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	736.0	708.3
Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	1,486.6	1,487.4
Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	156.9	158.2
Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	5,201.4	5,176.8
Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	782.9	784.7
Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	173.0	175.1
Credit card
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	75.2	74.6
Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	1,031.1	1,034.4
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	1,244.1	1,215.4
Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	133.9	136.2
Other Assets Especially Mentioned
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	159.5	155.1
Other Assets Especially Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	76.7	78.0
Other Assets Especially Mentioned | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	5.5	3.2
Other Assets Especially Mentioned | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	4.0	2.3
Other Assets Especially Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	7.9	2.4
Other Assets Especially Mentioned | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	17.4	7.9
Other Assets Especially Mentioned | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	6.7	3.9
Other Assets Especially Mentioned | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	6.4	4.3
Other Assets Especially Mentioned | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	107.2	94.1
Other Assets Especially Mentioned | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	0.7	0.8
Other Assets Especially Mentioned | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	0.4	0.4
Other Assets Especially Mentioned | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	1.1	1.2
Other Assets Especially Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	47.5	54.7
Other Assets Especially Mentioned | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	3.7	5.1
Substandard
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	212.4	217.2
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	92.8	96.4
Substandard | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	15.6	16.4
Substandard | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	1.0	1.7
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	3.6	3.6
Substandard | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	20.2	21.7
Substandard | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	11.4	12.5
Substandard | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	18.9	19.1
Substandard | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	143.3	149.7
Substandard | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	2.2	2.2
Substandard | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	0.8	0.7
Substandard | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	3.0	2.9
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	55.5	56.3
Substandard | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	10.6	8.3
Doubtful
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	24.6	28.2
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	8.0	10.3
Doubtful | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	0.0	0.0
Doubtful | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	0.5	0.5
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	3.5	3.5
Doubtful | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	4.0	4.0
Doubtful | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	0.9	1.9
Doubtful | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	0.0	0.0
Doubtful | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	12.9	16.2
Doubtful | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	0.2	0.3
Doubtful | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	0.2	0.2
Doubtful | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	0.4	0.5
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	10.8	11.1
Doubtful | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	0.5	0.4
Total Criticized Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	396.5	400.5
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	177.5	184.7
Total Criticized Loans | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	21.1	19.6
Total Criticized Loans | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	5.5	4.5
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	15.0	9.5
Total Criticized Loans | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	41.6	33.6
Total Criticized Loans | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	19.0	18.3
Total Criticized Loans | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	25.3	23.4
Total Criticized Loans | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	263.4	260.0
Total Criticized Loans | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	3.1	3.3
Total Criticized Loans | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	1.4	1.3
Total Criticized Loans | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	4.5	4.6
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	113.8	122.1
Total Criticized Loans | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans [Line Items]
Loans held for investment	$ 14.8	$ 13.8