Condensed Consolidated Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Assets
Accumulated depreciation	$ 242,630	$ 237,841
Equity
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	5,750,000	5,750,000
Preferred stock, shares outstanding (in shares)	5,750,000	5,750,000
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	125,000,000	125,000,000
Common stock, shares issued (in shares)	65,273,218	65,182,335
Common stock, shares outstanding (in shares)	65,273,218	65,182,335