Condensed Consolidated Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Assets
Accumulated depreciation	$ 213,735	$ 185,994
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)	65,134,056	59,047,001
Common stock, shares outstanding (in shares)	65,134,056	59,047,001