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Share-based compensation - 2011 Director Plan (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Share-based compensation |  |  |  |
Income tax benefit from compensation expense | $ 0.5 | $ 0.8 | $ 0.7 | $ 1.2
Common stock |  |  |  |
Share-based compensation |  |  |  |
Shares granted (in shares) | 37,747 | 35,000 | 38,821 | 35,770
Fair value measurement of shares granted and vested | $ 1.3 | $ 1.1 | $ 1.3 | $ 1.2
Income tax benefit from compensation expense | $ 0.3 | $ 0.4 | $ 0.3 | $ 0.5

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Number of share instruments newly issued under a share-based compensation plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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