Equity (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Changes in total common stockholders' equity    
Balance at the beginning of the period $ 626,626  
Share-based compensation expense 472  
Common stock dividends declared (8,007)  
Net income (loss) 1,575 (5,476)
Balance at the end of the period $ 620,666