Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue	$ 117,559	$ 109,384	$ 102,343
Operating expenses:
Direct	49,068	45,577	44,610
Selling, general and administrative	29,686	28,385	27,177
Depreciation and amortization	4,586	4,225	4,109
Total operating expenses	83,340	78,187	75,896
Operating income	34,219	31,197	26,447
Other income (expense):
Interest income	96	47	60
Interest expense	(82)	(190)	(220)
Other, net	50	302	1,073
Total other income	64	159	913
Income before income taxes	34,283	31,356	27,360
Provision for income taxes	11,340	10,838	9,750
Net income	22,943	20,518	17,610
Common Class A [Member]
Other income (expense):
Net income	$ 11,388	$ 10,178	$ 8,759
Basic earnings per share:
Basic earnings per share (in dollars per share)	$ 0.54	$ 0.49	$ 0.42
Diluted earnings per share:
Diluted earnings per share (in dollars per share)	$ 0.52	$ 0.48	$ 0.41
Weighted average shares and share equivalents outstanding
Weighted average shares and share equivalents outstanding, basic (in shares)	20,770	20,713	20,741
Weighted average shares and share equivalents outstanding, diluted (in shares)	21,627	21,037	20,981
Common Class B [Member]
Other income (expense):
Net income	$ 11,555	$ 10,341	$ 8,851
Basic earnings per share:
Basic earnings per share (in dollars per share)	$ 3.26	$ 2.93	$ 2.52
Diluted earnings per share:
Diluted earnings per share (in dollars per share)	$ 3.18	$ 2.88	$ 2.49
Weighted average shares and share equivalents outstanding
Weighted average shares and share equivalents outstanding, basic (in shares)	3,514	3,505	3,478
Weighted average shares and share equivalents outstanding, diluted (in shares)	3,603	3,560	3,522