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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Weighted average Common Shares outstanding
Weighted average Common Shares outstanding	100,384,606	96,353,818	99,928,738	96,182,753
Nonvested shares	(1,111,579)	(1,846,253)	(1,109,723)	(1,851,863)
Weighted average Common Shares—Basic	99,273,027	94,507,565	98,819,015	94,330,890
Dilutive effect of restricted stock	580,989	1,363,174	599,042	1,333,199
Dilutive effect of employee stock purchase plan	91,186	106,923	136,038	123,958
Weighted average Common Shares outstanding—Diluted	99,945,202	95,977,662	99,554,095	95,788,047
Net Income (Loss)
Income from continuing operations	$ 17,586	$ 9,005	$ 22,635	$ 16,482
Noncontrolling interests’ share in net income		(40)		(151)
Income from continuing operations attributable to common stockholders	17,586	8,965	22,635	16,331
Discontinued operations	330	(2,994)	663	(6,508)
NET INCOME ATTRIBUTABLE TO COMMON STOCKHOLDERS	$ 17,916	$ 5,971	$ 23,298	$ 9,823
Basic Earnings (Loss) Per Common Share
Income from continuing operations (in dollars per share)	$ 0.18	$ 0.09	$ 0.23	$ 0.17
Discontinued operations (in dollars per share)	0.00	(0.03)	0.01	(0.07)
Net income attributable to common stockholders (in dollars per share)	0.18	0.06	0.24	0.10
Diluted Earnings (Loss) Per Common Share
Income from continuing operations (in dollars per share)	0.18	0.09	0.23	0.17
Discontinued operations (in dollars per share)	0.00	(0.03)	0.00	(0.07)
Net income attributable to common stockholders (in dollars per share)	$ 0.18	$ 0.06	$ 0.23	$ 0.10