Stock option activity (Details) - Stock Options $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding
Shares Outstanding at Beginning of Period \| shares	124
Shares Exercised \| shares	(19)
Shares Outstanding at End of Period \| shares	105
Shares Exercisable \| shares	105
Weighted Average Exercise Price of Shares Outstanding at Beginning of Period \| $ / shares	$ 23.86
Weighted Average Exercise Price of Shares Exercised \| $ / shares	20.78
Weighted Average Exercise Price of Shares Outstanding at End of Period \| $ / shares	24.41
Weighted Average Exercise Price of Exercisable Shares \| $ / shares	$ 24.41
Weighted Average Remaining Contractual Term of Outstanding Options	1 year 5 months
Weighted Average Remaining Contractual Term of Exercisable Options	1 year 5 months
Options Outstanding, Intrinsic Value \| $	$ 0.5
Options, Exercisable, Intrinsic Value \| $	$ 0.5

The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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- Definition

Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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- Definition

Number of options outstanding, including both vested and non-vested options.

+ 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

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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- Definition

Number of fully vested and expected to vest exercisable options that may be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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- Definition

Weighted average price at which option holders acquired shares when converting their stock options into shares.

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No definition available.

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- Definition

Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

Number of share options (or share units) exercised during the current period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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-SubTopic 10
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