Stock Compensation Plans - Fair Value Assumptions for Employee Stock Options and Additional Stock Option Information (Details 4) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended				60 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2010	Dec. 31, 2015
Stock options
Cash received from the exercise of options	$ 1,095	$ 2,199	$ 4,066
Employee stock options
Assumptions used to calculate fair values
Weighted-average grant date fair value per stock option (in dollars per share)	$ 3.30	$ 4.48	$ 4.51
Expected dividend yield (as a percent)	0.96%	0.00%	0.00%
Expected volatility (as a percent)	19.00%		31.00%
Expected volatility, minimum (as a percent)		21.00%
Expected volatility, maximum (as a percent)		22.00%
Risk-free interest rates (as a percent)	1.48%		0.78%
Risk-free interest rates, minimum (as a percent)		1.51%
Risk-free interest rates, maximum (as a percent)		1.73%
Expected lives of options	4 years	4 years	4 years
Option term				10 years	7 years
Employee and nonemployee stock options
Stock options
Weighted-average grant date fair value options vested (in dollars per share)	$ 4.49	$ 4.80	$ 5.32
Total grant date fair value of options vested	$ 1,500	$ 1,300	$ 1,000
Total intrinsic value of stock options exercised	400	1,000	2,500
Cash received from the exercise of options	$ 1,100	$ 2,200	$ 4,100