Share-based compensation - 2011 Director Plan (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based compensation
Income tax benefit from compensation expense	$ 0.2	$ 0.4	$ 0.9	$ 1.5
Common stock
Share-based compensation
Shares granted (in shares)	0	0	38,821	35,770
Fair value measurement of shares granted and vested	$ 0.0	$ 0.0	$ 1.3	$ 1.2
Income tax benefit from compensation expense	$ 0.0	$ 0.0	$ 0.3	$ 0.5

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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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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Number of share instruments newly issued under a share-based compensation plan.

+ References

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-Name Accounting Standards Codification
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