Annual report pursuant to Section 13 and 15(d)

Shareholders' Equity (Capital Stock) (Details)

v3.8.0.1
Shareholders' Equity (Capital Stock) (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
May 21, 2013
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
May 22, 2017
May 18, 2016
May 17, 2016
May 31, 2015
May 22, 2015
Dec. 31, 2014
Nov. 21, 2014
Jun. 13, 2014
Dec. 31, 2013
Jun. 24, 2013
Class of Stock [Line Items]                            
Preferred stock, par value (in USD per share)   $ 0.0001 $ 0.0001                      
Common stock, shares authorized (in shares)   1,000,000,000 1,000,000,000     800,000,000       800,000,000     600,000,000  
Preferred stock, shares authorized (in shares)   50,000,000 50,000,000     50,000,000 25,000,000              
At the market preferred stock, shares authorized (in shares)         12,000,000       15,000,000   12,000,000 12,000,000   12,000,000
Preferred stock, dividends, per share, cash paid, monthly (in USD per share)   $ 0.1667                        
Series A Preferred Stock                            
Class of Stock [Line Items]                            
Preferred stock, shares outstanding, designated (in shares) 5,290,000             3,000,000   6,000,000       3,000,000
Preferred stock, par value (in USD per share) $ 0.0001 $ 0.0001 $ 0.0001                      
Preferred stock, shares authorized (in shares)   23,844,916 17,290,000                      
Preferred stock, dividend rate 8.00% 8.00% 8.00%                      
Preferred stock, liquidation preference (in USD per share) $ 25.00                          
Preferred stock, liquidation preference per month (in USD per share) $ 2.00                          
Redemption period 120 days                          
At the market shares sold (in shares)   1,167,159 1,161,895 3,586,790                    
Proceeds from at the market preferred stock   $ 29,052 $ 28,303 $ 86,352                    
Offering expenses   958 723 1,972                    
Dividends paid   32,482 $ 30,438 $ 28,108                    
Common Stock                            
Class of Stock [Line Items]                            
Dividends paid   $ 3,880                        
Common stock dividends paid, per share (in dollars per share)   $ 0.01