Summary of Significant Accounting Policies - Stock-Based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Stock-based compensation costs
Time period of historical volatility of stock used to estimate expected volatility	5 years 3 months 18 days
Accrued salaries and wages	$ 76,982		$ 111,252
Dividend yield (as a percent)	0.00%
Granted (in shares)	661,175	1,446,353
Weighted-average assumptions used in the Black-Scholes option-pricing model
Risk-free interest rate (as a percent)	2.26%	1.97%
Expected volatility (as a percent)	30.80%	30.67%
Dividend yield (as a percent)	0.00%
Weighted-average expected life	5 years 3 months 18 days	5 years 3 months 18 days
Employee stock options \| General and administrative expenses
Stock-based compensation costs
Compensation costs related to stock-based compensation	$ 2,900	$ 2,200
Phantom Share Units (PSUs)
Stock-based compensation costs
Accrued salaries and wages	800		4,800
Total compensation cost related to nonvested awards not yet recognized	$ 5,900
Period for recognition of unrecognized compensation cost	2 years 7 months 17 days
Compensation costs related to stock-based compensation	$ 100	4,300
Amounts paid on cash settled awards	$ 4,100	3,500
Vesting period	4 years
Stock appreciation rights (SARs)
Stock-based compensation costs
Accrued salaries and wages	$ 16,800		$ 24,000
Total compensation cost related to nonvested awards not yet recognized	$ 12,900
Period for recognition of unrecognized compensation cost	2 years 6 months 18 days
Compensation costs related to stock-based compensation	$ (4,000)	4,000
Amounts paid on cash settled awards	$ 3,000	$ 1,100
Vesting period	4 years

Represents the time period of historical volatility of stock used to estimate expected volatility, in periods.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

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-Topic 718
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-Section 50
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-Publisher FASB
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-Section 50
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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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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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- 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

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

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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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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Net number of share options (or share units) granted during the period.

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

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