The number of installments into which awards are divided upon meeting the criteria specified.

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The period during which the Company's average stock price is assessed in determining the vesting of an award to the recipient.

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Represents the period preceding the grant date during which the entity's average stock price is assessed in determining the vesting of an award to the recipient.

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Represents the percentage of matching units granted on the fourth anniversary of the grant date following vesting in annual installments on the first, second, and third anniversaries from grant date.

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Represents the percentage of matching units granted on the third anniversary of the grant date following vesting in annual installments on the first, second, and third anniversaries from grant date.

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Represents the percentage of matching units to be granted on grant date.

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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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Amount of deferred tax expense from write-off of the deferred tax asset related to deductible stock options at exercise.

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Unrecognized cost of unvested share-based compensation awards.

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

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The cash inflow associated with the amount received from holders exercising their stock options. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately.

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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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No definition available.

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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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Percentage of vesting of share-based compensation awards.

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

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Employee and Non-Employee Director Stock and Benefit Plans - Stock Based Awards (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) installment shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) shares
Employee and non-employee director stock and benefit plans
Stock options exercised \| shares		24,498	39,002
Share-based awards, additional disclosures
Total intrinsic value of stock options exercised		$ 220	$ 370
Weighted average grant date fair value of stock options granted during period, per share (in dollars per share) \| $ / shares		$ 16.18
Cash received from stock option exercises		$ 208	298
Number of installments \| installment	2
Stock options
Employee and non-employee director stock and benefit plans
Share-based compensation expense	$ 300	300	0
Tax benefit related to share-based compensation expense	$ 0	$ 100	$ 100
Weighted average remaining contractual term of stock options currently exercisable	6 years
Amount exercisable at the end of the period (in shares) \| shares	65,630		42,665
Tax shortfall from stock options exercises and cancellation			$ 600
Stock options exercised \| shares		42,665	71,455
Share-based awards, additional disclosures
Intrinsic value of outstanding stock options	$ 600
Intrinsic value of exercisable stock options	200
Unrecognized compensation costs	$ 300
Weighted average period over which unrecognized compensation costs are expected to be recognized	1 year
Restricted share awards
Employee and non-employee director stock and benefit plans
Share-based compensation expense	$ 19,600	$ 24,900	$ 16,400
Tax benefit related to share-based compensation expense	900	$ 8,900	$ 6,600
Share-based awards, additional disclosures
Unrecognized compensation costs	$ 25,800
Weighted average period over which unrecognized compensation costs are expected to be recognized	1 year 3 months 18 days
2007 Plan \| Restricted share awards \| Vesting on January 31, 2017
Share-based awards, additional disclosures
Vesting percent	33.33%
2007 Plan \| Restricted share awards \| Vesting on January 31, 2018
Share-based awards, additional disclosures
Vesting percent	66.67%
Equity Deferral Award Program (EDA)
Share-based awards, additional disclosures
Percentage of matching units granted on grant date prior to 2012	10.00%
Equity Deferral Award Program (EDA) \| Time-based award
Share-based awards, additional disclosures
Percentage of matching units granted on third anniversary of grant date	100.00%
Period preceding the grant date during which the entity's average stock price is assessed in determining the vesting of an award to the recipient	90 days
Period during which the Company's average stock price is assessed in determining the vesting of an award to the recipient	90 days
Equity Deferral Award Program (EDA) \| Market-based award
Share-based awards, additional disclosures
Period preceding the grant date during which the entity's average stock price is assessed in determining the vesting of an award to the recipient	90 days
Period during which the Company's average stock price is assessed in determining the vesting of an award to the recipient	90 days
Percentage of deferred compensation representing the match that vests on fourth anniversary	100.00%