Stock-Based Compensation- Employee Stock Purchase Plan (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2010
Employee Stock Purchase Plan ESPP Disclosures [Line Items]
Options To Purchase Common Stock Shares	0	0	0
Weighted average fair value of each option	$ 0	$ 0	$ 0
Employee Stock Purchase Plan [Member]
Employee Stock Purchase Plan ESPP Disclosures [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Market Price, Purchase Date	85.00%
Maximum amount of common stock that an employee can purchase under Employee Stock Purchase Plan in any calendar year	$ 25,000
Additional shares available for issuance	356,000			1,000,000
Options To Purchase Common Stock Shares	168,394	178,733	165,722
Weighted average fair value of each option	$ 7.91	$ 6.59	$ 9.86
Expected volatility	32.05%	39.45%	49.43%
Risk-free weighted average interest rate	0.07%	0.10%	0.10%
Vesting Period	0 years 6 months 0 days	0 years 6 months 0 days	0 years 6 months 0 days