Loans - Schedule of Loans by Class (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Loans and Leases Receivable
Loans held for investment	$ 8,930.7	$ 8,470.4
Mortgage loans held for sale	100.9	33.3
Total loans	9,031.6	8,503.7
Commercial
Loans and Leases Receivable
Loans held for investment	$ 3,484.7	3,235.4
Loans receivable, maturity	5 years
Residential
Loans and Leases Receivable
Loans held for investment	$ 244.1	242.8
Commercial
Loans and Leases Receivable
Loans held for investment	431.5	274.3
Total construction loans
Loans and Leases Receivable
Loans held for investment	977.7	838.7
Residential
Loans and Leases Receivable
Loans held for investment	$ 1,546.1	1,542.0
Loans receivable, maturity	15 years
Agricultural
Loans and Leases Receivable
Loans held for investment	$ 226.6	217.4
Total real estate loans
Loans and Leases Receivable
Loans held for investment	6,235.1	5,833.5
Indirect consumer
Loans and Leases Receivable
Loans held for investment	784.6	787.8
Other consumer
Loans and Leases Receivable
Loans held for investment	179.0	200.6
Credit card consumer
Loans and Leases Receivable
Loans held for investment	81.6	81.8
Total consumer loans
Loans and Leases Receivable
Loans held for investment	1,045.2	1,070.2
Commercial
Loans and Leases Receivable
Loans held for investment	1,371.3	1,310.3
Agricultural
Loans and Leases Receivable
Loans held for investment	279.1	254.8
Other, including overdrafts
Loans and Leases Receivable
Loans held for investment	0.0	1.6
Loans held for investment
Loans and Leases Receivable
Loans held for investment	8,930.7	8,470.4
Mortgage loans originated for sale
Loans and Leases Receivable
Mortgage loans held for sale	100.9	33.3
Total loans
Loans and Leases Receivable
Total loans	9,031.6	8,503.7
Home equity line of credit
Loans and Leases Receivable
Loans held for investment	$ 423.5	$ 409.5
Loans receivable, loan to value limit	80.00%