Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 73.0	$ 72.1	$ 76.2
Provision charged (credited) to operating expense	(13.9)	(8.6)	(11.0)
Less loans charged-off	(23.6)	(19.7)	(22.8)
Add back recoveries of loans previously charged-off	9.7	12.0	7.7
Ending balance	73.0	73.0	72.1
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				$ 3.6	$ 6.8	$ 10.8
Collectively evaluated for impairment				69.4	66.2	61.3
Ending balance	73.0	73.0	76.2	73.0	73.0	72.1
Total loans:
Individually evaluated for impairment				64.7	64.8	83.2
Collectively evaluated for impairment				8,866.0	8,405.6	7,484.5
Total loans held for investment				8,930.7	8,470.4	7,567.7
Allowance for loans losses				1.0	0.8
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	31.0	31.7	28.6
Provision charged (credited) to operating expense	(1.3)	(0.7)	(6.0)
Less loans charged-off	(3.5)	(3.7)	(4.3)
Add back recoveries of loans previously charged-off	2.7	3.7	1.4
Ending balance	28.9	31.0	31.7
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				1.7	1.3	6.2
Collectively evaluated for impairment				27.2	29.7	25.5
Ending balance	31.0	31.7	31.7	28.9	31.0	31.7
Total loans:
Individually evaluated for impairment				41.1	41.8	58.3
Collectively evaluated for impairment				6,194.0	5,791.7	5,118.5
Total loans held for investment				6,235.1	5,833.5	5,176.8
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	8.7	8.7	7.7
Provision charged (credited) to operating expense	(10.6)	(6.8)	(8.1)
Less loans charged-off	(13.0)	(11.3)	(11.3)
Add back recoveries of loans previously charged-off	3.6	4.5	4.2
Ending balance	9.9	8.7	8.7
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				0.0	0.0	0.0
Collectively evaluated for impairment				9.9	8.7	8.7
Ending balance	9.9	8.7	8.7	9.9	8.7	8.7
Total loans:
Individually evaluated for impairment				0.0	0.0	0.0
Collectively evaluated for impairment				1,045.2	1,070.2	1,034.4
Total loans held for investment				1,045.2	1,070.2	1,034.4
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	31.3	30.5	38.1
Provision charged (credited) to operating expense	(4.5)	(1.9)	(2.9)
Less loans charged-off	(6.6)	(4.7)	(6.8)
Add back recoveries of loans previously charged-off	3.4	3.6	2.1
Ending balance	32.6	31.3	30.5
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				1.7	5.2	4.4
Collectively evaluated for impairment				30.9	26.1	26.1
Ending balance	31.3	31.3	38.1	32.6	31.3	30.5
Total loans:
Individually evaluated for impairment				17.3	19.9	23.8
Collectively evaluated for impairment				1,354.0	1,290.4	1,191.6
Total loans held for investment				1,371.3	1,310.3	1,215.4
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	2.0	1.2	1.8
Provision charged (credited) to operating expense	(0.1)	(0.6)	(0.2)
Less loans charged-off	(0.5)	0.0	(0.4)
Add back recoveries of loans previously charged-off	0.0	0.2	0.0
Ending balance	1.6	2.0	1.2
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				0.2	0.3	0.2
Collectively evaluated for impairment				1.4	1.7	1.0
Ending balance	2.0	1.2	1.2	1.6	2.0	1.2
Total loans:
Individually evaluated for impairment				6.3	3.1	1.1
Collectively evaluated for impairment				272.8	251.7	135.1
Total loans held for investment				279.1	254.8	136.2
Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	0.0	0.0	0.0
Provision charged (credited) to operating expense	0.0	0.0	0.0
Less loans charged-off	0.0	0.0	0.0
Add back recoveries of loans previously charged-off	0.0	0.0	0.0
Ending balance	0.0	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				0.0	0.0	0.0
Collectively evaluated for impairment				0.0	0.0	0.0
Ending balance	$ 0.0	$ 0.0	$ 0.0	0.0	0.0	0.0
Total loans:
Individually evaluated for impairment				0.0	0.0	0.0
Collectively evaluated for impairment				0.0	1.6	4.9
Total loans held for investment				$ 0.0	$ 1.6	$ 4.9