CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
CONSOLIDATED BALANCE SHEETS
Vessels, accumulated depreciation	$ 221,646	$ 197,521
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, authorized shares	225,000,000	225,000,000
Common stock, issued shares	83,267,426	82,960,194
Common stock, outstanding shares	83,267,426	82,960,194
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred Stock, Shares Issued	0	0
Preferred stock, outstanding shares	0	0