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	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator for basic net income per common share [Abstract]
Net income	$ 79,589	$ 82,873	$ 108,325
Preferred stock dividends	(17,442)	(15,372)	(15,160)
Earnings participation of unvested equity awards	(150)	(140)	(123)
Numerator for basic net income per common share	$ 61,997	$ 67,361	$ 93,042
Denominator for basic net income per common share [Abstract]
Average number of shares of common stock outstanding (in shares)	96,023	95,957	95,817
Average unvested stock awards outstanding (in shares)	(305)	(301)	(308)
Denominator for basic net income per common share (in shares)	95,718	95,656	95,509
Basic net income per common share (in dollars per share)	$ 0.65	$ 0.70	$ 0.97
Numerator for diluted net income per common share [Abstract]
Numerator for basic net income per common share	$ 61,997	$ 67,361	$ 93,042
Denominator for diluted net income per common share [Abstract]
Denominator for basic net income per common share (in shares)	95,718	95,656	95,509
Net effect of dilutive equity awards (in shares)	125	163	192
Denominator for diluted net income per common share (in shares)	95,843	95,819	95,701
Diluted net income per common share (in dollars per share)	$ 0.65	$ 0.70	$ 0.97