Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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- Definition

Unrecognized cost of unvested share-based compensation awards.

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

Discount rate from fair value on purchase date that participants pay for shares.

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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.

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The highest quantity of shares an employee can purchase under the plan per period.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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- Definition

The number of shares purchased on the open market during the period for issuance to employees under the plan.

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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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- Definition

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

Purchase price of common stock expressed as a percentage of its fair value.

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Stock-based compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	8 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award
Share-based compensation		$ 10.1	$ 4.7	$ 0.0
Maximum number of shares per employees		5,000
Average price of shares purchased (dollars per share)	$ 0.00	$ 27.38
RSU, DSU, PSU
Share-based Compensation Arrangement by Share-based Payment Award
Share-based compensation		$ 8.4	4.0
Unrecognized stock based compensation	$ 8.2	$ 12.6	$ 8.2
Unrecognized stock based compensation expense, recognition period (years)		1 year 2 months	1 year 7 months
ESPP
Share-based Compensation Arrangement by Share-based Payment Award
Share-based compensation		$ 0.2	$ 0.0
Maximum shares reserve for equity awards		250,000
Shares available for grant		243,261
Shares purchased under the ESPP		6,739
Discount on common stock (as a percentage)		15.00%
Purchase price of common stock (as a percentage)		85.00%
Average price of shares purchased (dollars per share)		$ 48.09
Stock Option
Share-based Compensation Arrangement by Share-based Payment Award
Share-based compensation		$ 1.5	0.7
Expiration period (in years)		10 years
Unrecognized stock based compensation	$ 1.4	$ 2.3	$ 1.4
Unrecognized stock based compensation expense, recognition period (years)		1 year 5 months	2 years 2 months
Stock Option \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period on stock options (years)		1 year
Stock Option \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period on stock options (years)		3 years
RSUs and DSUs \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period on stock options (years)	1 year
RSUs and DSUs \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period on stock options (years)	3 years
Performance Shares (PSUs)
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period on stock options (years)		3 years
WestRock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award
Share-based compensation		$ 0.0	$ 0.5	$ 2.3
2016 Omnibus Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award
Maximum shares reserve for equity awards		4,000,000
Shares available for grant		3,265,453