CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets
Accumulated depreciation	$ 282,812	$ 237,841
Equity
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	5,750,000	5,750,000
Preferred stock, shares outstanding	5,750,000	5,750,000
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares issued	65,285,614	65,182,335
Common stock, shares outstanding	65,285,614	65,182,335