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Equity - Class of Stock Disclosures (Details) - USD ($)
 1 Months Ended 3 Months Ended 12 Months Ended
Apr. 30, 2016
Mar. 31, 2016
Feb. 29, 2016
Jan. 31, 2016
Mar. 31, 2015
Feb. 28, 2015
Jan. 31, 2015
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Dec. 31, 2013
Equity                      
Net proceeds               $ 14,637,000      
Dividend Distributions                      
Declared   $ 20,347,000     $ 18,934,000     20,347,000 $ 18,934,000    
Paid               20,347,000 18,934,000    
Dividends declared and paid per common share per month (in dollars per share)   $ 0.18 $ 0.18 $ 0.18 $ 0.17 $ 0.17 $ 0.17        
Accumulated other comprehensive income                      
Accumulated other comprehensive income   $ 18,000           $ 18,000   $ 47,000  
Subsequent Event | Dividend Payable, April 29, 2016                      
Dividend Distributions                      
Dividends per share declared (in dollars per share) $ 0.18                    
Subsequent Event | Dividend Payable, May 31, 2016                      
Dividend Distributions                      
Dividends per share declared (in dollars per share) 0.18                    
Subsequent Event | Dividend Payable, June 30, 2016                      
Dividend Distributions                      
Dividends per share declared (in dollars per share) $ 0.18                    
Shelf Registration                      
Equity                      
Automatic shelf registration statement, term (in years)                     P3Y
Series C Preferred Stock                      
Equity                      
Preferred shares outstanding   0           0   2,000,000  
Dividend Rate (as a percent)                   8.50%  
Number of shares of common stock to be issued upon conversion                   2,000,000  
Conversion price per share                   $ 19.25  
Shares converted                   2,000,000  
Dividend Distributions                      
Declared         $ 818,000       818,000    
Paid                 $ 818,000    
Common Stock                      
Equity                      
Number of shares repurchased               30,910 4,609    
Dividend Distributions                      
Declared   $ 20,347,000     $ 18,116,000     $ 20,347,000 $ 18,116,000    
Paid               $ 20,347,000 $ 18,116,000    
Common Stock | Equity Distribution Agreement                      
Equity                      
Shares common stock sold               332,619      
Net proceeds               $ 14,637,000      
Reclassification of accumulated costs to additional paid in capital               310,000      
Maximum offering capacity under shelf registration statement                   $ 200,000,000  
Amount available under effective shelf registration statement   185,102,000           185,102,000      
Common Stock | Shelf Registration                      
Equity                      
Maximum offering capacity under shelf registration statement                     $ 800,000,000
Amount available under effective shelf registration statement   $ 575,100,000           $ 575,100,000