Shareholders' Equity (Details)			9 Months Ended
	Apr. 19, 2018 USD ($)	Feb. 11, 2014 USD ($)	Sep. 30, 2018 vote $ / shares shares	Sep. 30, 2017 $ / shares shares	Dec. 31, 2017 $ / shares	Apr. 24, 2012 USD ($)
Class of Stock [Line Items]
Amount authorized under stock repurchase program | $						$ 10,000,000
Stock repurchase program, additional authorized amount | $	$ 30,000,000	$ 20,000,000
Common Class A
Class of Stock [Line Items]
Economic interest held in company by class of stock (in hundredths)			25.00%		26.30%
Par value (in dollars per share)			$ 0.01		$ 0.01
Percentage of outstanding shares that represents rights of holders to receive distribution (in hundredths)			100.00%
Common stock purchased and retired (in shares) | shares			742,255	79,717
Purchased under repurchase authorization at an average price per share (in dollars per share)			$ 10.10	$ 8.88
Common Class B
Class of Stock [Line Items]
Economic interest held in company by class of stock (in hundredths)			75.00%		73.70%
Par value (in dollars per share)			$ 0.000001		$ 0.000001
Voting rights (in votes) | vote			5
Percentage of stock holding that will entitle holders to one vote (in hundredths)			20.00%
Voting rights when class of stock constitutes less than 20% of all shares outstanding (in votes) | vote			1
Common stock purchased and retired (in shares) | shares			3,870	2,897
Purchased under repurchase authorization at an average price per share (in dollars per share)			$ 10.67	$ 11.11