Stockholders' Equity - Computation of basic and diluted earnings (loss) per common share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Weighted average Common Shares outstanding
Weighted average Common Shares outstanding	125,217,707	116,528,181	125,192,557	116,499,667
Nonvested shares	(1,933,073)	(1,807,082)	(1,921,489)	(1,801,448)
Weighted average Common Shares—Basic	123,284,634	114,721,099	123,271,068	114,698,219
Dilutive effect of nonvested share-based awards		861,037		796,283
Dilutive effect of employee stock purchase plan	36,371	92,228	53,092	102,327
Weighted average Common Shares outstanding—Diluted	123,321,005	115,674,364	123,324,160	115,596,829
Net Income	$ 37,729	$ 25,224	$ 46,908	$ 57,070
Dividends paid on nonvested share-based awards	(582)		(1,160)
Net income applicable to common stockholders	$ 37,147	$ 25,224	$ 45,748	$ 57,070
Basic earnings per common share - Net income (in dollars per share)	$ 0.30	$ 0.22	$ 0.37	$ 0.50
Diluted earnings per common share - Net income (in dollars per share)	$ 0.30	$ 0.22	$ 0.37	$ 0.49