Loans - Schedule of Recorded Investment in Criticized Loans by Class and Credit Quality Indicator (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	$ 8,930.7	$ 8,470.4
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,484.7	3,235.4
Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	302.1	321.6
Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	244.1	242.8
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	431.5	274.3
Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	977.7	838.7
Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,546.1	1,542.0
Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	226.6	217.4
Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	6,235.1	5,833.5
Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	784.6	787.8
Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	179.0	200.6
Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,045.2	1,070.2
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,371.3	1,310.3
Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	279.1	254.8
Other Assets Especial Mentioned
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	157.5	178.8
Other Assets Especial Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	84.7	102.5
Other Assets Especial Mentioned | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3.8	5.0
Other Assets Especial Mentioned | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.9	2.8
Other Assets Especial Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1.7	1.7
Other Assets Especial Mentioned | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	6.4	9.5
Other Assets Especial Mentioned | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2.6	3.0
Other Assets Especial Mentioned | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	14.3	9.0
Other Assets Especial Mentioned | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	108.0	124.0
Other Assets Especial Mentioned | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.2	0.7
Other Assets Especial Mentioned | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.4	0.3
Other Assets Especial Mentioned | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.6	1.0
Other Assets Especial Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	40.4	39.4
Other Assets Especial Mentioned | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	8.5	14.4
Substandard
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	224.0	201.6
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	97.3	87.4
Substandard | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1.9	7.0
Substandard | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2.2	2.0
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1.5	3.9
Substandard | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	5.6	12.9
Substandard | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	7.8	10.8
Substandard | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	26.6	24.0
Substandard | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	137.3	135.1
Substandard | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2.9	2.1
Substandard | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.8	0.8
Substandard | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3.7	2.9
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	60.3	45.8
Substandard | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	22.7	17.8
Doubtful
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	5.9	20.9
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.8	2.9
Doubtful | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1.0	3.3
Doubtful | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.0	0.4
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.0	0.0
Doubtful | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1.0	3.7
Doubtful | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.3	0.7
Doubtful | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.0	0.1
Doubtful | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2.1	7.4
Doubtful | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.0	0.1
Doubtful | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.1	0.1
Doubtful | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.1	0.2
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3.6	11.8
Doubtful | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.1	1.5
Total Criticized Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	387.4	401.3
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	182.8	192.8
Total Criticized Loans | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	6.7	15.3
Total Criticized Loans | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3.1	5.2
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3.2	5.6
Total Criticized Loans | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	13.0	26.1
Total Criticized Loans | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	10.7	14.5
Total Criticized Loans | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	40.9	33.1
Total Criticized Loans | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	247.4	266.5
Total Criticized Loans | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3.1	2.9
Total Criticized Loans | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1.3	1.2
Total Criticized Loans | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	4.4	4.1
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	104.3	97.0
Total Criticized Loans | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	$ 31.3	$ 33.7