Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Jun. 08, 2018	Jun. 07, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Preferred stock, par value	$ 0.001			$ 0.001
Preferred stock, shares authorized	100,000,000			100,000,000
Preferred stock, shares issued	0			0
Preferred Stock, Shares Outstanding	0			0
Total stockholders’ equity	$ 399,259			$ 396,774	$ 390,144	$ 397,694
Common stock
Common stock, par value	$ 0.001
Common stock, shares authorized	200,000,000
Common stock, shares issued	35,870,348
Common stock, shares outstanding	35,870,348
Treasury stock, shares	0
Common Stock - Class A
Common stock, par value		$ 0.001	$ 0.001	$ 0.001
Common stock, shares authorized				200,000,000
Common stock, shares issued				35,003,004
Common stock, shares outstanding				35,003,004
Treasury stock, shares				5,197,551
Common Stock - Class B
Common stock, par value	$ 0.001			$ 0.001
Common stock, shares authorized	0			50,000,000
Common stock, shares issued	0			8,049,029
Common stock, shares outstanding	0			8,049,029
Treasury stock, shares	0			8,049,029