Investments in Qualified Affordable Housing Projects, Federal Historic Projects and New Market Tax Credits (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Variable Interest Entity [Line Items]
Expense related to qualified affordable housing projects	$ 3.4	$ 2.6	$ 1.3
Remaining investment in qualified affordable housing projects	51.4	29.5
Equity method investments	17.3	10.9
Remaining unfunded equity contributions	48.1	16.0
Impairment on tax credit projects	7.9	$ 0.0
Operating expense
Variable Interest Entity [Line Items]
Impairment on tax credit projects	$ 9.3

Amount of expenses incurred on qualified affordable housing project investments accounted for using the equity method.

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- Definition

Amount of impairment on tax credit projects in qualified affordable housing projects.

+ References

No definition available.

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- Definition

Amount of remaining investment to qualified affordable housing project investments.

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No definition available.

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- Definition

The reporting entity's unfunded obligation amount as a result of its involvement with the Variable Interest Entity (VIE).

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No definition available.

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- Definition

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66022249&loc=d3e33749-111570

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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