Stock-Based Compensation Plans (Details) (USD $)	12 Months Ended							12 Months Ended														12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 The 2007 plan	Dec. 31, 2011 The 2007 plan	Dec. 31, 2010 The 2007 plan	May 24, 2007 The 2007 plan	Dec. 31, 2012 The 2007 plan Restricted stock	Dec. 31, 2011 The 2007 plan Restricted stock	Dec. 31, 2010 The 2007 plan Restricted stock	Dec. 31, 2012 The 2007 plan Restricted stock vesting over 4 years	Dec. 31, 2011 The 2007 plan Restricted stock vesting over 4 years	Dec. 31, 2010 The 2007 plan Restricted stock vesting over 4 years	Dec. 31, 2012 The 2007 plan Restricted stock vesting on the first anniversary date from date of issuance	Dec. 31, 2011 The 2007 plan Restricted stock vesting on the first anniversary date from date of issuance	Dec. 31, 2010 The 2007 plan Restricted stock vesting on the first anniversary date from date of issuance	Dec. 31, 2012 1996 Stock Options/Stock Issuance Plan	Dec. 31, 2011 1996 Stock Options/Stock Issuance Plan	Dec. 31, 2010 1996 Stock Options/Stock Issuance Plan	Dec. 31, 2009 1996 Stock Options/Stock Issuance Plan	Jun. 30, 2006 1996 Stock Options/Stock Issuance Plan	Dec. 31, 2012 Employee Stock Purchase Plan	Dec. 31, 2011 Employee Stock Purchase Plan	Dec. 31, 2010 Employee Stock Purchase Plan	May 20, 2010 Employee Stock Purchase Plan
Stock-based compensation plans
Stock compensation expense (in dollars)	$ 3,144,000	$ 3,173,000	$ 2,678,000	$ 3,092,000	$ 3,108,000	$ 2,603,000													$ 14,000			$ 52,000	$ 65,000	$ 61,000
Number of shares authorized							2,000,000														3,025,000				250,000
Number of shares awarded								1,728,156
Remaining average vesting period for recognition of unrecognized compensation expense								2 years 6 months
Unrecognized compensation expense (in dollars)								6,600,000
Intrinsic value of unvested awards (in dollars)								9,800,000
Number Outstanding
Balance at the beginning of the period (in shares)				651,294	575,791	558,304		651,294	575,791	558,304
Shares granted				311,040	347,024	212,010					283,000	324,924	190,375	28,040	22,100	21,635
Shares cancelled				(8,988)	(27,477)			(8,988)	(27,477)
Shares vested				(270,692)	(244,044)	(194,523)		(270,692)	(244,044)	(194,523)
Balance at the end of the period (in shares)				682,654	651,294	575,791		682,654	651,294	575,791
Weighted Average Grant Date Fair Value
Balance at the beginning of the period (in dollars per share)				$ 11.61	$ 10.90	$ 11.14
Shares granted (in dollars per share)				$ 8.55	$ 12.45	$ 11.19																$ 3.19	$ 3.40	$ 3.40
Shares cancelled (in dollars per share)				$ 12.52	$ 11.34
Shares vested (in dollars per share)				$ 11.33	$ 11.14	$ 11.89
Balance at the end of the period (in dollars per share)				$ 11.61	$ 11.61	$ 10.90
Aggregate Value
Balance at the beginning of the period (in dollars)				7,564,276	6,276,184	6,217,214
Shares granted (in dollars)				2,659,170	4,318,920	2,371,619
Shares cancelled (in dollars)				(112,521)	(311,596)
Shares vested (in dollars)				(3,067,357)	(2,719,232)	(2,312,649)
Balance at the end of the period (in dollars)				7,043,568	7,564,276	6,276,184
Maximum percentage of cash compensation allowed to be deducted for the purchase of common stock by eligible employees																						10.00%
Maximum number of shares to be purchased by employee in one calendar year																						1,000
Maximum amount of shares to be purchased by employee in one calendar year (in dollars)																						25,000
Purchase price expressed as a percentage of the market price of the common stock on the purchase date or on the date of entry																						85.00%
Shares issued																						18,482	19,983
Options exercised (in shares)																	(306,653)	(369,310)	(206,146)
Stock options outstanding (in shares)																	136,928	443,581	812,891	1,019,788
Intrinsic value of outstanding stock options (in dollars)																	$ 781,692	$ 2,484,009	$ 5,064,940	$ 8,486,579