Investment Securities - Proceeds from Calls and Sales, Realized Gains and Losses and Interest Income (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($) item	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($) item
Proceeds from calls and sales of investment securities
Proceeds from calls of available for sale securities	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Proceeds from sales of available for sale securities	0.0	0.0	0.0	0.0
Debt Securities, Available-for-sale, Realized Gain (Loss) [Abstract]
Gross realized gains on sales of investment securities	0.0	0.0	0.0	0.0
Gross realized losses on sales of investment securities	0.0	0.0	0.0	0.0
Provision for income taxes related to net realized gains on sale of investment securities	0.0	0.0	0.0	0.0
Interest income from taxable and nontaxable investment securities
Taxable interest income	25.1	$ 24.2	81.2	$ 75.7
Non-taxable interest income	$ 0.1		$ 0.4
Non-taxable investment securities owned \| item		0		0

Represents the number of non-taxable investment securities owned.

No definition available.

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- Definition

The cash inflow associated with calls on debt securities classified as available-for-sale securities.

+ References

No definition available.

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No definition available.

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- Definition

Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of operating interest income, including amortization and accretion of premiums and discounts, on securities exempt from state, federal and other income tax.

+ References

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-SubTopic 220
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Amount of operating interest income, including amortization and accretion of premiums and discounts, on securities subject to state, federal and other income tax.

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- Definition

Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

+ References

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Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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