Stock-Based Compensation Plans (Details) (USD $)	12 Months Ended							12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 The 2007 plan	Dec. 31, 2012 The 2007 plan	Dec. 31, 2011 The 2007 plan	May 24, 2007 The 2007 plan	Dec. 31, 2013 The 2007 plan Restricted stock	Dec. 31, 2012 The 2007 plan Restricted stock	Dec. 31, 2011 The 2007 plan Restricted stock	Dec. 31, 2013 The 2007 plan Restricted stock Awards vesting on first anniversary	Dec. 31, 2012 The 2007 plan Restricted stock Awards vesting on first anniversary	Dec. 31, 2011 The 2007 plan Restricted stock Awards vesting on first anniversary	Dec. 31, 2013 The 2007 plan Restricted stock Awards vesting over four years	Dec. 31, 2012 The 2007 plan Restricted stock Awards vesting over four years	Dec. 31, 2011 The 2007 plan Restricted stock Awards vesting over four years	Dec. 31, 2013 Employee Stock Purchase Plan	Dec. 31, 2012 Employee Stock Purchase Plan	Dec. 31, 2011 Employee Stock Purchase Plan	May 20, 2010 Employee Stock Purchase Plan
Stock-based compensation plans
Stock compensation expense (in dollars)	$ 3,439,000	$ 3,144,000	$ 3,173,000	$ 3,393,000	$ 3,092,000	$ 3,108,000											$ 46,000	$ 52,000	$ 65,000
Number of shares authorized							2,000,000													250,000
Number of shares awarded								1,879,564
Remaining average vesting period for recognition of unrecognized compensation expense								2 years 2 months 12 days
Unrecognized compensation expense (in dollars)								5,200,000
Intrinsic value of unvested awards (in dollars)								8,900,000
Number Outstanding
Balance at the beginning of the period (in shares)				682,654	651,294	575,791		682,654	651,294	575,791
Shares granted				151,408	311,040	347,024					21,408	28,040	22,100	130,000	283,000	324,924
Shares cancelled				(60,110)	(8,988)	(27,477)		(60,110)	(8,988)	(27,477)
Shares vested				(258,822)	(270,692)	(244,044)		(258,822)	(270,692)	(244,044)
Balance at the end of the period (in shares)				515,130	682,654	651,294		682,654	682,654	651,294
Vesting period														4 years	4 years	4 years
Weighted Average Grant Date Fair Value
Balance at the beginning of the period (in dollars per share)				$ 12.83	$ 11.79	$ 10.90
Shares granted (in dollars per share)				$ 14.25	$ 13.68	$ 12.78											$ 3.19	$ 3.19	$ 3.40
Shares cancelled (in dollars per share)				$ 12.41	$ 12.52	$ 11.34
Shares vested (in dollars per share)				$ 12.01	$ 11.33	$ 11.14
Balance at the end of the period (in dollars per share)				$ 13.71	$ 12.83	$ 11.79
Aggregate Value
Balance at the beginning of the period (in dollars)				8,756,990	7,680,578	6,276,184
Shares granted (in dollars)				2,158,011	4,256,290	4,435,222
Shares cancelled (in dollars)				(745,876)	(112,521)	(311,596)
Shares vested (in dollars)				(3,108,651)	(3,067,357)	(2,719,232)
Balance at the end of the period (in dollars)				7,060,474	8,756,990	7,680,578
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																	14,026	18,482