Employee Stock Purchase Plan (Tables)

12 Months Ended

Dec. 31, 2011

Employee Stock Purchase Plan [Abstract]

Assumptions used in estimating the fair value of employees' purchase rights

	Six Month Offering Periods Ending
	December  31, 2011			June  30, 2011			December  31, 2010			June  30, 2010			December  31, 2009			June  30, 2009
Expected volatility		45.86	%		46.58	%		47.44	%		48.10	%		48.83	%		47.32	%
Expected term		6 months			6 months			6 months			6 months			6 months			6 months
Risk-free interest rate		0.10	%		0.19	%		0.22	%		0.20	%		0.35	%		0.27	%
Divided yield		None			None			None			None			None			None

Per share fair value of employees' purchase rights in each of the six month offering periods

	Six Month Offering Periods Ending
	December  31, 2011		June  30, 2011		December  31, 2010		June  30, 2010		December  31, 2009		June  30, 2009
Grant-date fair value	$	19.65	$	12.34	$	9.20	$	7.69	$	7.71	$	8.36