Stock-Based Compensation - Restricted Stock, PSUs, and SARs (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-based compensation costs
Compensation costs related to stock-based compensation	$ 7,800	$ 6,900	$ 8,200
Total compensation cost related to nonvested awards not yet recognized	12,200	11,600	11,200
Accrued salaries and wages	$ 111,252	$ 92,238
Maximum
Stock-based compensation costs
Vesting period	4 years
Restricted Stock
Stock-based compensation
Outstanding at the beginning of the period (in shares)	175,886
Awarded (in shares)	176,865
Released (in shares)	(68,257)
Canceled (in shares)	(16,839)
Outstanding at the end of the period (in shares)	267,655	175,886
Stock-based compensation costs
Total compensation cost related to nonvested awards not yet recognized	$ 2,400	$ 1,700	2,400
Employee stock options
Stock-based compensation costs
Total compensation cost related to nonvested awards not yet recognized	9,800	9,900	8,800
Phantom Share Units (PSUs)
Stock-based compensation costs
Compensation costs related to stock-based compensation	11,900	8,500	14,100
Total compensation cost related to nonvested awards not yet recognized	$ 5,700
Period for recognition of unrecognized compensation cost	2 years 4 months 10 days
Accrued salaries and wages	$ 4,800	5,600
Amounts paid on cash settled awards	$ 12,700	10,700	14,500
Phantom Share Units (PSUs) \| Minimum
Stock-based compensation costs
Vesting period	3 years
Phantom Share Units (PSUs) \| Maximum
Stock-based compensation costs
Vesting period	4 years
Stock appreciation rights (SARs)
Stock-based compensation costs
Compensation costs related to stock-based compensation	$ 21,900	2,400	5,100
Total compensation cost related to nonvested awards not yet recognized	$ 17,500
Period for recognition of unrecognized compensation cost	2 years 6 months
Accrued salaries and wages	$ 24,000	7,300
Vesting period	4 years
Amounts paid on cash settled awards	$ 6,200	$ 3,300	$ 3,400

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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Total of the carrying values as of the balance sheet date of obligations incurred through that date and payable for obligations related to services received from employees, such as accrued salaries and bonuses, payroll taxes and fringe benefits. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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Aggregate cash paid by the entity during the period to settle equity instruments granted under equity-based payment arrangements.

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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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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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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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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 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.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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