Represents the number of years used in the calculation of total return to shareholders.

No definition available.

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- Definition

Represents the high end of range of payout depending on achievement of the goals.

+ References

No definition available.

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- Definition

Represents the low end of range of payout depending on achievement of the goals.

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No definition available.

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- Definition

Represents the number of shares that could be issued upon vesting of share-based payment awards and achievement of performance goals.

+ References

No definition available.

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- Definition

Represents the number of authorized but unissued shares that could be issued under the equity-based compensation plan.

+ References

No definition available.

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Award Performance Period

+ References

No definition available.

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- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Disclosure of the aggregate tax benefit realized from the exercise of stock options and the conversion of similar instruments during the annual period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (j)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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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

No definition available.

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- Definition

Number of additional shares authorized for issuance under an established share-based compensation plan.

+ References

No definition available.

+ Details

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- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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Share-based compensation - Narrative (Details) - USD ($) $ in Millions		1 Months Ended	3 Months Ended		9 Months Ended
	Mar. 01, 2014	Jul. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based compensation
Income tax benefit from compensation expense			$ 0.4	$ 0.5	$ 1.5	$ 1.2
Restricted stock units
Share-based compensation
Cash paid to directors in lieu of stock awards					3.4	2.7
Income tax benefit from compensation expense					1.1	0.9
Unrecognized share based compensation			4.8		$ 4.8
Weighted average period for recognition of unrecognized compensation cost (in years)					2 years 7 months 6 days
Long-term Incentive Plan
Share-based compensation
Payment award, low end of range (as a percent)					0.00%
Payment award, high end of range (as a percent)					200.00%
Award performance period (in years)					3 years
LTIP linked to TRS
Share-based compensation
Cash paid to directors in lieu of stock awards					$ 1.9
Unrecognized share based compensation			1.0		$ 1.0
Weighted average period for recognition of unrecognized compensation cost (in years)					2 years 3 months 18 days
Measurement period for total return to shareholders (in years)					3 years
Tax benefits related to awards vested					$ 0.7
LTIP awards linked to other performance conditions
Share-based compensation
Cash paid to directors in lieu of stock awards					4.2	3.6
Income tax benefit from compensation expense					1.6	$ 1.4
Unrecognized share based compensation			$ 3.4		$ 3.4
Weighted average period for recognition of unrecognized compensation cost (in years)					2 years 3 months 18 days
Equity and Incentive Plan
Share-based compensation
Number of additional shares authorized (in shares)	1,500,000
Shares available for future issuance (in shares)			3,300,000		3,300,000
Number of share issuable upon vesting and achievement of performance goals (in shares)			400,000		400,000
Nonemployee Director Stock Plan
Share-based compensation
Shares available for future grant (in shares)			85,428		85,428
Cash paid to directors in lieu of stock awards		$ 0.2