Stock-Based Compensation (Details) (USD $)	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Stock-Based Compensation
Stock-based compensation expense (in dollars)	$ 4,400,000	$ 3,700,000
Employee Stock Purchase Plan
Employee Stock Purchase Plan
Number of shares issued	0	0
Shares of common stock reserved for issuance under the ESPP	1,250,000
Shares available for issuance			1,103,937
Unrecognized compensation expense (in dollars)	216,000
Weighted-average period of recognition of unrecognized stock-based compensation expense	2 months
Weighted-average fair values and assumptions used in calculation of fair value
Expected volatility (as a percent)	51.00%	47.00%
Risk-free interest rate (as a percent)	0.14%	0.05%
Expected life	6 months	6 months
Weighted-average grant date fair value (in dollars per share)	$ 7.50	$ 7.16
Stock Option Grants
Stock Option Grants
Common stock issued in connection with the exercise of stock options (in shares)	195,524	224,239
Weighted-average exercise price of common stock issued (in dollars per share)	$ 16.15	$ 16.61
Weighted-average fair values and assumptions used in calculation of fair value
Dividend yield (as a percent)	0.00%	0.00%
Expected volatility (as a percent)	46.00%	46.00%
Risk-free interest rate (as a percent)	1.26%	1.23%
Expected life	6 years 7 months 24 days	6 years 9 months 29 days
Weighted-average grant date fair value (in dollars per share)	$ 13.19	$ 13.97
Stock Option Grants \| Employees
Stock Option Grants
Options granted to purchase shares of common stock to employees (in shares)	317,500	504,500
Restricted stock units
Stock awards other than options
Stock awards (in shares)	254,598	351,548
Grant-date fair value (in dollars)	7,300,000	10,100,000
Number of shares of common stock to be received for each restricted stock unit	1
Shares of common stock issued in connection with the vesting of the stock award	117,434	59,514
Weighted-average fair values and assumptions used in calculation of fair value
Weighted-average grant date fair value (in dollars per share)	$ 26.44	$ 23.16
Restricted stock units \| Employees
Stock awards other than options
Fraction of shares awarded that vest per year (as a percent)	33.00%
Vesting period	3 years
Restricted stock \| Outside directors
Stock awards other than options
Stock awards (in shares)	1,835	1,572
Grant-date fair value (in dollars)	$ 50,000	$ 40,000
Weighted-average fair values and assumptions used in calculation of fair value
Weighted-average grant date fair value (in dollars per share)	$ 27.24	$ 25.39

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The total fair value at grant-date for nonvested equity-based awards issued 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 fraction of shares awarded that vest per year.

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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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Aggregate unrecognized cost of share-based awards, other than options, made to employees under an equity-based compensation plan, that have yet to vest.

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Period the instrument, asset or liability is expected to be outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price.

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Risk-free interest rate assumption used in valuing an instrument.

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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 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 weighted average fair value at grant date for nonvested equity-based awards issued 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 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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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.

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

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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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Number of share instruments newly issued under a share-based compensation plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Number of shares issued during the period as a result of an employee stock purchase plan.

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