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Capital Stock Dividends Declared (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Class of Stock [Line Items]    
Total Amount $ 21,922 $ 21,742
Regular Cash Dividend    
Class of Stock [Line Items]    
Dividends declared per common share (usd per share) $ 0.25 $ 0.25