Acquisition (Details 2) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pro forma financial information
Revenues	$ 59,481	$ 57,177
Income before income taxes	24,418	22,824
Net income	$ 15,395	$ 13,529
Basic net income per common share	$ 0.42	$ 0.37
Diluted net income per common share	$ 0.41	$ 0.35