Share-based payments - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of vested shares	$ 2.1	$ 4.2
Compensation costs	$ 2.1	$ 4.8	$ 5.3
Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	1 year
Shares granted in relation to the annual director retainer compensation (in shares)	13,700	14,300	13,000
Fair value of vested shares	$ 0.6	$ 0.6	$ 0.6
Time-Based Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			3 years
Performance-Based Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			3 years
Restricted Stock and Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate fair values at grant date	$ 5.1	$ 4.8	$ 5.7
Share-based incentives, shares issued	122,400	110,600	133,000
Average grant price (in usd per share)	$ 42.02	$ 43.39	$ 42.59
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based incentives, shares issued	13,702
Average grant price (in usd per share)	$ 41.26
Shares granted in relation to the annual director retainer compensation (in shares)	33,532
Tax benefits related to vesting of awards	$ 0.5	$ 1.1	$ 1.2
Compensation costs not yet recognized	$ 4.1
Compensation costs not yet recognized, period for recognition	1 year 8 months 12 days

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

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Amount of tax benefit for recognition of expense of award under share-based payment arrangement.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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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.

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