Stock-Based Compensation (Details) (USD $)	3 Months Ended	6 Months Ended
Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Stock-Based Compensation
Stock-based compensation expense	$ 3,700,000	$ 3,000,000	$ 7,400,000	$ 5,900,000
Stock Option Grants
Stock Option Grants
Common stock issued in connection with the exercise of stock options (in shares)	308,049	189,187	532,288	425,546
Weighted-average exercise price of common stock issued (in dollars per share)	$ 15.07	$ 12.66	$ 15.72	$ 12.94
Weighted-average fair values and assumptions used in calculation of fair value
Dividend yield (as a percent)			0.00%
Expected volatility (as a percent)	46.00%	46.00%	46.00%	47.00%
Risk-free interest rate (as a percent)	1.05%	1.97%	1.23%	2.30%
Expected life	6 years 9 months 29 days	5 years 8 months 16 days	6 years 9 months 29 days	6 years 2 months 12 days
Weighted-average fair value (in dollars per share)	$ 16.25	$ 11.65	$ 14.17	$ 11.18
Stock options granted to employees and outside directors
Stock Option Grants
Options granted to purchase shares of common stock to employees (in shares)	62,000	69,500	566,500	548,900
Restricted Stock Units and Restricted Stock
Restricted Stock Units/Restricted Stock
Stock awards (in shares)	35,344		386,892
Restricted stock units awarded, total grant-date fair value	1,100,000		11,200,000
Shares of common stock issued in connection with the vesting of RSUs and restricted stock	3,880		64,966
Number of shares of common stock to be received for each restricted stock unit (in shares)			1
Weighted-average fair value (in dollars per share)	$ 28.48		$ 23.42
Weighted-average fair values and assumptions used in calculation of fair value
Weighted-average fair value (in dollars per share)	$ 28.48		$ 23.42
Restricted stock units \| Employees
Restricted Stock Units/Restricted Stock
Stock awards (in shares)	34,040		384,016
Fraction of shares awarded that vest per year (in shares)			0.33
Vesting period			3 years
Restricted stock \| Outside directors
Restricted Stock Units/Restricted Stock
Stock awards (in shares)	1,304	2,030	2,876	2,030
Restricted stock units awarded, total grant-date fair value		53,000		53,000
Weighted-average fair value (in dollars per share)		$ 26.15		$ 26.15
Weighted-average fair values and assumptions used in calculation of fair value
Weighted-average fair value (in dollars per share)		$ 26.15		$ 26.15
Employee Stock Purchase Plan
Restricted Stock Units/Restricted Stock
Weighted-average fair value (in dollars per share)	$ 7.48		$ 7.32
Employee Stock Purchase Plan
Number of shares issued	69,856		69,856
Shares of common stock reserved for issuance under the ESPP (in shares)	1,250,000		1,250,000
Shares available for issuance	1,180,144		1,180,144
Unrecognized compensation expense	$ 431,000		$ 431,000
Weighted-average period of recognition of unrecognized stock-based compensation expense			5 months
Weighted-average fair values and assumptions used in calculation of fair value
Expected volatility (as a percent)	44.00%		46.00%
Risk-free interest rate (as a percent)	0.07%		0.06%
Expected life	6 months		6 months
Weighted-average fair value (in dollars per share)	$ 7.48		$ 7.32

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

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