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Subsequent Events (Details) - $ / shares
1 Months Ended
Apr. 24, 2018
Jan. 31, 2018
Subsequent Events:    
Dividend declared (in dollars per share)   $ 1.50
Subsequent events    
Subsequent Events:    
Dividend declared (in dollars per share) $ 1.57  
Dividend declared, date Apr. 24, 2018  
Dividend payable, date Jun. 09, 2018  
Shareholders of record, date May 10, 2018  
Dividend per share increase (in dollars per share) $ 0.07  
Dividend per share increase (as a percent) 5.00%