Quarterly report pursuant to Section 13 or 15(d)

Shareholders' Equity (Details)

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Shareholders' Equity (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Jun. 30, 2018
Jun. 30, 2017
Jul. 21, 2016
Shareholders' Equity [Line Items]                  
Stock Repurchase Program, Number of Shares Authorized to be Repurchased                 3,000,000
Stock Repurchased and Retired During Period, Shares 132,880       42,055   497,166 246,864  
Stock Repurchased and Retired During Period, In Total Dollars $ 8,172,000       $ 1,999,000   $ 28,165,000 $ 11,995,000  
Stock repurchased, common stock acquired, average cost per share $ 61.50       $ 47.54   $ 56.65 $ 48.59  
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased 1,321,499           1,321,499    
Common Stock [Member]                  
Shareholders' Equity [Line Items]                  
Dividends per share (in dollars per share) $ 0.15 $ 0.15 $ 0.15 $ 0.15 $ 0.15 $ 0.15 $ 0.30 $ 0.30