NET INCOME PER COMMON SHARE - Components of Computation of Basic and Diluted Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator for basic net income per common share [Abstract]
Net income	$ 17,549	$ 21,630	$ 40,250	$ 48,983
Preferred stock dividends	(4,018)	(3,843)	(7,882)	(7,669)
Earnings participation of unvested equity awards	(40)	(39)	(80)	(83)
Numerator for basic net income per common share	$ 13,491	$ 17,748	$ 32,288	$ 41,231
Denominator for basic net income per common share [Abstract]
Average number of shares of common stock outstanding (in shares)	96,063	95,947	96,062	95,930
Average unvested stock awards outstanding (in shares)	(307)	(299)	(307)	(299)
Denominator for basic net income per common share (in shares)	95,756	95,648	95,755	95,631
Basic net income per common share (in dollars per share)	$ 0.14	$ 0.19	$ 0.34	$ 0.43
Numerator for diluted net income per common share [Abstract]
Numerator for basic net income per common share	$ 13,491	$ 17,748	$ 32,288	$ 41,231
Denominator for diluted net income per common share [Abstract]
Denominator for basic net income per common share (in shares)	95,756	95,648	95,755	95,631
Net effect of dilutive equity awards (in shares)	160	138	140	135
Denominator for diluted net income per common share (in shares)	95,916	95,786	95,895	95,766
Diluted net income per common share (in dollars per share)	$ 0.14	$ 0.19	$ 0.34	$ 0.43