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Stock-Based Compensation, Stock Options Exercised (Details) - Stock Options [Member] - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock-Based Compensation [Abstract]      
Cash proceeds from stock options exercised $ 26,980 $ 15,707 $ 13,041
Tax benefit realized for stock options exercised 6,457 3,840 4,451
Intrinsic value of stock options exercised $ 42,749 $ 30,587 $ 12,085