Stock-Based Compensation Plans - Restricted stock activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
The 2007 plan
Number Outstanding
Balance at the beginning of the period (in shares)	299,335	396,595	525,356
Shares granted	215,603	136,645	146,440
Shares forfeited	(10,999)	(20,377)
Shares vested	(175,817)	(213,528)	(275,201)
Balance at the end of the period (in shares)	328,122	299,335	396,595
Weighted Average Grant Date Fair Value
Balance at the beginning of the period (in dollars per share)	$ 20.24	$ 17.98	$ 13.71
Shares granted (in dollars per share)	24.89	21.55	18.53
Shares cancelled (in dollars per share)	24.38	17.79
Shares vested (in dollars per share)	19.61	17.12	15.87
Balance at the end of the period (in dollars per share)	$ 23.49	$ 20.24	$ 17.98
Aggregate Value
Balance at the beginning of the period (in dollars)	$ 6,058,336	$ 7,131,200	$ 8,786,611
Shares granted (in dollars)	5,365,731	2,944,941	2,713,159
Shares forfeited (in dollars)	(268,200)	(362,536)
Shares vested (in dollars)	(3,448,628)	(3,655,269)	(4,368,570)
Balance at the end of the period (in dollars)	$ 7,707,239	$ 6,058,336	$ 7,131,200
Number of shares available	350,784
The 2007 plan | Restricted stock
Aggregate Value
Balance at the end of the period (in dollars)	$ 8,200,000
Remaining average vesting period for recognition of unrecognized compensation expense	1 year 8 months 12 days
Unrecognized compensation expense (in dollars)	$ 5,100,000
Extended expiration period	5 years
Employee Stock Purchase Plan
Weighted Average Grant Date Fair Value
Shares granted (in dollars per share)	$ 7.39	$ 6.35	$ 6.17
Aggregate Value
Shares issued	11,485	11,014	10,374
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%
1996 Plan
Aggregate Value
Stock options exercised (in shares)			49,000
Total intrinsic value of options exercised (in dollars)			$ 300,000
Number of shares available	0