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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Weighted average Common Shares outstanding
Weighted average Common Shares outstanding	129,865,985	125,233,462	128,348,638	125,206,342
Nonvested shares	(1,775,911)	(1,933,653)	(1,777,743)	(1,925,589)
Weighted average Common Shares—Basic	128,090,074	123,299,809	126,570,895	123,280,753
Dilutive effect of employee stock purchase plan	78,610	52,147	85,690	54,763
Weighted average Common Shares outstanding—Diluted	128,168,684	123,351,956	126,656,585	123,335,516
Net Income	$ 2,601	$ 6,548	$ 11,975	$ 53,456
Dividends paid on nonvested share-based awards	(534)	(580)	(1,603)	(1,740)
Net income applicable to common stockholders	$ 2,067	$ 5,968	$ 10,372	$ 51,716
Basic earnings per common share (in dollars per share)	$ 0.02	$ 0.05	$ 0.08	$ 0.42
Diluted earnings per common share (in dollars per share)	$ 0.02	$ 0.05	$ 0.08	$ 0.42