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RELATED-PARTY TRANSACTIONS - Transactions Involving Underwriting Services (Details) - USD ($)
 3 Months Ended 12 Months Ended
Mar. 08, 2016
Feb. 08, 2016
Apr. 08, 2015
Apr. 06, 2015
May. 21, 2014
Mar. 31, 2016
Dec. 31, 2015
Related Party Transaction [Line Items]              
Expenses from transactions with related parties       $ 263,000      
Expenses from transactions with related parties, net of related expenses       221,000      
Common Stock              
Related Party Transaction [Line Items]              
Issuance of stock shares 4,850,000            
Senior Notes | 5.25% Senior Notes Due April 15, 2025              
Related Party Transaction [Line Items]              
Aggregate principal amount issued       $ 175,000,000      
Stated interest rate       5.25%      
Director | Underwriting Fees              
Related Party Transaction [Line Items]              
Expenses from transactions with related parties   $ 944,000 $ 590,000        
Expenses from transactions with related parties, net of related expenses   $ 849,000 $ 515,000        
Director | Underwriting Fees | Common Stock              
Related Party Transaction [Line Items]              
Expenses from transactions with related parties $ 1,000,000       $ 521,000    
Expenses from transactions with related parties, net of related expenses $ 846,000       $ 481,000    
Series E Preferred Stock              
Related Party Transaction [Line Items]              
Depository shares to preferred stock ratio   2.50%          
Non cumulative preferred stock, dividend rate   7.00%       7.00% 7.00%
Preferred stock, liquidation preference per share (in dollars per share)   $ 1,000       $ 1,000 $ 1,000
Offering price per share   $ 25          
Series D Preferred Stock              
Related Party Transaction [Line Items]              
Depository shares to preferred stock ratio     2.50%        
Non cumulative preferred stock, dividend rate     7.375%     7.375% 7.375%
Preferred stock, liquidation preference per share (in dollars per share)     $ 1,000     $ 1,000 $ 1,000
Offering price per share     $ 25        
Underwritten Public Offering              
Related Party Transaction [Line Items]              
Depository shares issued   5,000,000 4,600,000        
Offering price per share   $ 25 $ 25        
Underwritten Public Offering | Common Stock              
Related Party Transaction [Line Items]              
Issuance of stock shares 5,577,500       5,922,500    
Underwritten Public Offering | Series E Preferred Stock              
Related Party Transaction [Line Items]              
Depository shares issued   5,000,000          
Depository shares to preferred stock ratio   2.50%          
Preferred stock, liquidation preference per share (in dollars per share)   $ 1,000          
Underwritten Public Offering | Series D Preferred Stock              
Related Party Transaction [Line Items]              
Depository shares to preferred stock ratio     2.50%        
Preferred stock, liquidation preference per share (in dollars per share)     $ 1,000