Loans - Schedule of Loans by Class (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Loans and Leases Receivable
Loans held for investment	$ 8,470.4	$ 7,567.7
Mortgage loans held for sale	33.3	46.6
Total loans	8,503.7	7,614.3
Commercial
Loans and Leases Receivable
Loans held for investment	$ 3,235.4	2,822.9
Loans receivable, maturity	5 years
Land acquisition & development
Loans and Leases Receivable
Loans held for investment	$ 321.6	348.7
Residential
Loans and Leases Receivable
Loans held for investment	242.8	240.2
Commercial
Loans and Leases Receivable
Loans held for investment	274.3	119.4
Total construction loans
Loans and Leases Receivable
Loans held for investment	838.7	708.3
Residential
Loans and Leases Receivable
Loans held for investment	$ 1,542.0	1,487.4
Loans receivable, maturity	15 years
Agricultural
Loans and Leases Receivable
Loans held for investment	$ 217.4	158.2
Total real estate loans
Loans and Leases Receivable
Loans held for investment	5,833.5	5,176.8
Indirect consumer
Loans and Leases Receivable
Loans held for investment	787.8	784.7
Other consumer
Loans and Leases Receivable
Loans held for investment	200.6	175.1
Credit card consumer
Loans and Leases Receivable
Loans held for investment	81.8	74.6
Total consumer loans
Loans and Leases Receivable
Loans held for investment	1,070.2	1,034.4
Commercial
Loans and Leases Receivable
Loans held for investment	1,310.3	1,215.4
Agricultural
Loans and Leases Receivable
Loans held for investment	254.8	136.2
Other, including overdrafts
Loans and Leases Receivable
Loans held for investment	1.6	4.9
Loans held for investment
Loans and Leases Receivable
Loans held for investment	8,470.4	7,567.7
Total loans
Loans and Leases Receivable
Total loans	8,503.7	7,614.3
Home equity line of credit
Loans and Leases Receivable
Loans held for investment	$ 409.5	$ 397.0
Loans receivable, loan to value limit	80.00%