Stockholders Equity - Schedule of Shares Offered and Increase in Shares Outstanding in Follow-on Offerings (Details) - USD ($) $ in Thousands	1 Months Ended						48 Months Ended
	Aug. 31, 2018	Mar. 31, 2018	Jul. 31, 2017	Jan. 31, 2017	Nov. 30, 2014	Apr. 30, 2014	Sep. 30, 2018
Class A Common Stock
Class Of Stock [Line Items]
Total Shares Offered	5,000,000	5,000,000	6,000,000	5,750,000	6,325,000		28,075,000
Total Increase in Shares Outstanding	4,685,217	4,689,295	5,680,903	5,356,876	4,511,058		24,923,349
Class B Common Stock
Class Of Stock [Line Items]
Total Increase in Shares Outstanding						36,158,698
Stock Purchased/Surrendered	4,685,217	4,689,295	5,680,903	5,356,876	4,507,453		24,919,744
Purchase Cost	$ 158	$ 135	$ 128	$ 101	$ 28		$ 550