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Capital Stock Quarterly Dividend (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Quarterly Cash Dividend [Line Items]                      
Common stock dividends $ 21,837 $ 21,834 $ 21,816 $ 21,742 $ 21,612 $ 22,510 $ 22,959 $ 23,215 $ 87,229 $ 90,296  
Regular Cash Dividend                      
Quarterly Cash Dividend [Line Items]                      
Dividends declared (usd per share) $ 0.25 $ 0.25 $ 0.25 $ 0.25 $ 0.25 $ 0.25 $ 0.25 $ 0.25 $ 1 $ 1.0 $ 0.75