Common stock (Restricted stock awards and restricted stock units) (Details) - Restricted Stock Units (RSUs) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options	$ 30.2
Outstanding, Number of Shares	1,550,646	1,807,180
Outstanding, Wtd Ag Grant Date Fair Value	$ 28.94	$ 23.13
Vesting percentage on 1st anniversary of grant (percent)	33.33%
Vesting percentage on 2nd anniversary of grant (percent)	33.33%
Vesting percentage on 3rd anniversary of grant (percent)	33.34%

Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

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 (i)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ 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 (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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- Definition

The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

+ 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 (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=109197908&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.

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No definition available.

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other Than Options, Award Vesting Percentage, Two Years from Grant Date

+ References

No definition available.

+ Details

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other Than Options, Vesting Percentage, One Year from Grant Date

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No definition available.

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- Definition

Share Based Compensation Arrangement By Share Based Payment Award, Equity Instruments Other Than Options, Vesting Percentage, Three Years From Grant Date

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No definition available.

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