Stockholders' Equity - Computation of basic and diluted earnings (loss) per common share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Weighted average Common Shares outstanding
Weighted average Common Shares outstanding	125,233,462	120,895,835	125,206,342	117,981,159
Nonvested shares	(1,933,653)	(1,797,686)	(1,925,589)	(1,800,180)
Weighted average Common Shares—Basic	123,299,809	119,098,149	123,280,753	116,180,979
Dilutive effect of employee stock purchase plan	52,147	83,242	54,763	96,260
Weighted average Common Shares outstanding—Diluted	123,351,956	119,181,391	123,335,516	116,277,239
Net Income	$ 6,548	$ 3,173	$ 53,456	$ 60,243
Dividends paid on nonvested share-based awards	(580)	(538)	(1,740)	(1,609)
Net income applicable to common stockholders	$ 5,968	$ 2,635	$ 51,716	$ 58,634
Basic earnings per common share (in dollars per share)	$ 0.05	$ 0.02	$ 0.42	$ 0.50
Diluted earnings per common share (in dollars per share)	$ 0.05	$ 0.02	$ 0.42	$ 0.50