NET INCOME PER COMMON SHARE - Components of Computation of Basic and Diluted Net Income Per Common Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Numerator for basic net income per common share [Abstract]
Net income	$ 33,957	$ 38,391
Preferred stock dividends	(3,742)	(3,238)
Earnings participation of unvested equity awards	(34)	(24)
Numerator for basic net income per common share	30,181	35,129
Denominator for basic net income per common share [Abstract]
Weighted average common stock outstanding (in shares)	95,825	95,777
Average unvested stock awards outstanding (in shares)	(356)	(428)
Denominator for basic net income per common share (in shares)	95,469	95,349
Basic net income per common share (in dollars per share)	$ 0.32	$ 0.37
Numerator for diluted net income per common share [Abstract]
Numerator for basic net income per common share	$ 30,181	$ 35,129
Denominator for diluted net income per common share [Abstract]
Denominator for basic net income per common share	95,469	95,349
Net effect of dilutive equity awards	205	189
Denominator for diluted net income per common share (in shares)	95,674	95,538
Diluted net income per common share (in dollars per share)	$ 0.32	$ 0.37