CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Vessels, accumulated depreciation (in dollars)	$ 128,583	$ 109,235
Common stock, par value (in dollars per share)	$ 0.01
Common stock, authorized shares	225,000,000
Common stock, issued shares	79,990,334
Common stock, outstanding shares	79,990,334
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	25,000,000	5,000,000
Preferred Stock, Shares Issued	0	0
Preferred stock, outstanding shares	0	0
Class A common stock
Common stock, par value (in dollars per share)		$ 0.01
Common stock, authorized shares		50,000,000
Common stock, issued shares		11,270,196
Common stock, outstanding shares		11,270,196
Class B common stock
Common stock, par value (in dollars per share)		$ 0.01
Common stock, authorized shares		30,000,000
Common stock, issued shares		22,002,998
Common stock, outstanding shares		22,002,998