Shareholders' Equity (Details)	3 Months Ended			6 Months Ended
	Mar. 31, 2019 shares	Jun. 30, 2018 shares	Mar. 31, 2018 shares	Jun. 30, 2019 vote $ / shares shares	Jun. 30, 2018 $ / shares shares	Dec. 31, 2018 $ / shares	Apr. 19, 2018 USD ($)	Feb. 11, 2014 USD ($)	Apr. 24, 2012 USD ($)
Class of Stock [Line Items]
Amount authorized under stock repurchase program | $							$ 30,000,000	$ 20,000,000	$ 10,000,000
Common Class A
Class of Stock [Line Items]
Economic interest held in company by class of stock (in hundredths)				25.50%		26.40%
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	533,369	232,340	293,130	533,369	525,470
Purchased under repurchase authorization at an average price per share (in dollars per share)				$ 8.83	$ 10.43
Common Class B
Class of Stock [Line Items]
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	95,739		3,870	95,739	3,870
Purchased under repurchase authorization at an average price per share (in dollars per share)				$ 6.01	$ 10.67
Pzena Investment Management, LLC | Common Class B
Class of Stock [Line Items]
Economic interest held in company by class of stock (in hundredths)				74.50%		73.60%