Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Dec. 31, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 72.1	$ 76.2
Provision charged to operating expense	2.1	1.7
Less loans charged-off	5.2	3.8
Add back recoveries of loans previously charged-off	4.5	2.1
Ending balance	73.5	76.2
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Loans individually evaluated for impairment			$ 10.6	$ 10.8
Loans collectively evaluated for impairment			62.9	61.3
Allowance for loan losses	72.1	76.2	73.5	72.1
Loans held for investment:
Individually evaluated for impairment			76.5	83.2
Individually evaluated for impairment			7,538.0	7,484.5
Total loans			7,614.5	7,567.7
Allowance for credit losses due to subsequent impairment			1.0
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	31.6	28.6
Provision charged to operating expense	1.8	4.4
Less loans charged-off	0.8	0.4
Add back recoveries of loans previously charged-off	1.2	0.5
Ending balance	33.8	33.1
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Loans individually evaluated for impairment			6.0	6.2
Loans collectively evaluated for impairment			27.8	25.4
Allowance for loan losses	31.6	28.6	33.8	31.6
Loans held for investment:
Individually evaluated for impairment			54.1	58.3
Individually evaluated for impairment			5,147.3	5,118.5
Total loans			5,201.4	5,176.8
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	8.7	7.7
Provision charged to operating expense	0.7	2.0
Less loans charged-off	2.7	2.6
Add back recoveries of loans previously charged-off	1.1	1.2
Ending balance	7.8	8.3
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Loans individually evaluated for impairment			0.0	0.0
Loans collectively evaluated for impairment			7.8	8.7
Allowance for loan losses	8.7	7.7	7.8	8.7
Loans held for investment:
Individually evaluated for impairment			0.0	0.0
Individually evaluated for impairment			1,031.1	1,034.4
Total loans			1,031.1	1,034.4
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	30.6	38.1
Provision charged to operating expense	(0.7)	(4.1)
Less loans charged-off	1.7	0.8
Add back recoveries of loans previously charged-off	2.0	0.4
Ending balance	30.2	33.6
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Loans individually evaluated for impairment			4.5	4.4
Loans collectively evaluated for impairment			25.7	26.2
Allowance for loan losses	30.6	38.1	30.2	30.6
Loans held for investment:
Individually evaluated for impairment			20.4	23.8
Individually evaluated for impairment			1,223.7	1,191.6
Total loans			1,244.1	1,215.4
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	1.2	1.8
Provision charged to operating expense	0.3	(0.6)
Less loans charged-off	0.0	0.0
Add back recoveries of loans previously charged-off	0.2	0.0
Ending balance	1.7	1.2
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Loans individually evaluated for impairment			0.1	0.2
Loans collectively evaluated for impairment			1.6	1.0
Allowance for loan losses	1.2	1.8	1.7	1.2
Loans held for investment:
Individually evaluated for impairment			2.0	1.1
Individually evaluated for impairment			131.9	135.1
Total loans			133.9	136.2
Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	0.0	0.0
Provision charged to operating expense	0.0	0.0
Less loans charged-off	0.0	0.0
Add back recoveries of loans previously charged-off	0.0	0.0
Ending balance	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Loans individually evaluated for impairment			0.0	0.0
Loans collectively evaluated for impairment			0.0	0.0
Allowance for loan losses	$ 0.0	$ 0.0	0.0	0.0
Loans held for investment:
Individually evaluated for impairment			0.0	0.0
Individually evaluated for impairment			4.0	4.9
Total loans			$ 4.0	$ 4.9

Loans And Leases Receivable Held For Investment, Evaluated for Impairment

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Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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For loans and debt securities, 1) for which at acquisition it was probable that the investor would be unable to collect all contractually required payments receivable, and 2) for which it is further probable that the investor will not be able to collect all cash flows expected at acquisition (plus any additional cash flows expected due to changes in estimate after acquisition), the allowance for credit losses recorded to reflect the impairment since acquisition.

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The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

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The balance of financing receivables that were collectively evaluated for impairment.

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The balance of financing receivables that were individually evaluated for impairment.

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Amount of expense related to estimated loss from loan and lease transactions.

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