Consolidated balance sheets (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Treasury stock, shares	256,895
Class A Common stock, $0.01 par value, 180,000,000 shares authorized; 38,845,797 issued and 38,588,902 outstanding at September 30, 2018 and 38,956,250 shares issued and outstanding at December 31, 2017
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	180,000,000	180,000,000
Common stock, shares issued	38,845,797	38,956,250
Common stock, shares outstanding	38,588,902	38,956,250
Class B Common stock, $0.01 par value, 20,000,000 shares authorized and 7,871,512 shares issued and outstanding at September 30, 2018 and December 31, 2017
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	20,000,000	20,000,000
Common stock, shares issued	7,871,512	7,871,512
Common stock, shares outstanding	7,871,512	7,871,512