Annual report pursuant to Section 13 and 15(d)

Shareholders' Equity (Capital Stock) (Details)

v3.19.1
Shareholders' Equity (Capital Stock) (Details) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 3 Months Ended 12 Months Ended
May 21, 2013
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
May 22, 2017
May 18, 2016
May 17, 2016
Dec. 31, 2015
May 31, 2015
May 22, 2015
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 $ 0.0001                      
Common stock, shares authorized (in shares)     1,000,000,000       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     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                        
Dividends declared             $ 8,794 $ 3,880                      
Series A Preferred Stock                                      
Class of Stock [Line Items]                                      
Preferred stock, shares outstanding, designated (in shares) 5,290,000                       6,000,000 3,000,000         3,000,000
Preferred stock, par value (in USD per share) $ 0.0001   $ 0.0001       $ 0.0001 $ 0.0001                      
Preferred stock, shares authorized (in shares)     23,844,916       23,844,916 23,844,916                      
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                      
Proceeds from at the market preferred stock             $ 29,052 $ 28,303                      
Offering expenses             958 723                      
Dividends paid             $ 34,031 $ 32,482 $ 30,438                    
Common Stock                                      
Class of Stock [Line Items]                                      
Common stock dividends paid, per share (in dollars per share)     $ 0.0125 $ 0.0125 $ 0.01 $ 0.01                          
Subsequent event | Common Stock                                      
Class of Stock [Line Items]                                      
Common stock dividends paid, per share (in dollars per share)   $ 0.0125