Stock-Based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized under stock compensation plan	7.8
Shares available for grant under stock compensation plan	4.9
Share-based compensation expense	$ 4.6	$ 4.8	$ 6.7
Tax benefit from share-based compensation expense	$ 1.1	$ 2.2	$ 2.2
Vesting period	3 years
Weighted average fair value of options granted (usd per share)	$ 7.00	$ 4.25	$ 6.12
Stock options, vested and expected to vest, outstanding (shares)	2.2
Stock options, vested and expected to vest, weighted average exercise price (usd per share)	$ 10.91
Stock options, exercisable at end of year (shares)	1.6	1.6	1.4
Stock options, expected to vest at end of year (shares)	0.6
Stock options outstanding, aggregate intrinsic value	$ 20.8
Stock options exercisable, aggregate intrinsic value	16.9
Stock options exercised during the period, total intrinsic value	2.3	$ 0.4	$ 0.8
Tax benefit realized from stock option exercises during the period	0.9	0.1	0.2
Proceeds from stock compensation activity	1.6	0.5	1.0
Excess tax benefit from stock-based compensation	0.0	0.0	1.6
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense related to share-based compensation plans	2.2	2.1	2.0
Unrecognized compensation cost	$ 2.3
Weighted-average recognition period for unrecognized compensation cost	1 year 8 months 12 days
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense related to share-based compensation plans	$ 1.6	1.2	1.2
Unrecognized compensation cost	$ 1.2
Weighted-average recognition period for unrecognized compensation cost	2 years 1 month 6 days
Grant-date fair value of awards vested	$ 1.5	1.3	1.3
PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense related to share-based compensation plans	0.8	1.5	3.5
Unrecognized compensation cost, performance share units	$ 2.0
Weighted-average recognition period for unrecognized compensation cost	2 years
Grant-date fair value of awards vested	$ 0.3	$ 3.7	$ 3.7
Minimum \| PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awarded percentage	0.00%
Maximum \| PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awarded percentage	200.00%
Performance-based Restricted Stock Units, 2015 Grants \| PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Performance share units performance period	3 years
Performance-based Restricted Stock Units, 2016 Grants \| PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Performance share units performance period	3 years
Performance-based Restricted Stock Units, 2017 Grants \| PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Performance share units performance period	3 years

Share Based Compensation Arrangement by Share Based Payment Award, Performance Period

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Share-based Compensation Arrangement by Share-based Payment Award, Options, Expected to Vest, Outstanding, Number

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

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Unrecognized cost of unvested options awarded to employees as compensation.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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Disclosure of the aggregate tax benefit realized from the exercise of stock options and the conversion of similar instruments during the annual period.

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Amount of cash outflow for realized tax benefit related to deductible compensation cost reported on the entity's tax return for equity instruments in excess of the compensation cost for those instruments recognized for financial reporting purposes.

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The total cash inflow associated with the amount received from holders to acquire the entity's shares under incentive and share awards, including stock option exercises. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately.

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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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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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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 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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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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

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As of the balance sheet date, the number of shares into which fully vested and expected to vest stock options outstanding can be converted under the option plan.

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As of the balance sheet date, the weighted-average exercise price for outstanding stock options that are fully vested or expected to vest.

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Amount of capitalized and expensed compensation cost from equity-based compensation arrangements.

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

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