Share-Based Compensation Plans - Deferred Stock (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$ 27.8	$ 28.7	$ 27.1
Deferred stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$ 1.9	$ 1.4	$ 1.1
Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	145,000	167,000	232,000
Forfeited (in shares)	23,000	37,000	14,000
Share-based compensation	$ 13.3	$ 12.3	$ 13.8
In lieu of annual cash retainer \| Deferred stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	5,551	3,647	3,714
Additional compensation for board service \| Deferred stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	10,152	9,857	8,388
Share-based payment award vesting period	1 year
Expiration period	3 years
Additional compensation for board service \| Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	6,345	9,792	9,966
Forfeited (in shares)	1,666

The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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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.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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-Name Accounting Standards Codification
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