Stock Based Compensation (Stock Option Activity) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options, Beginning of Year | shares	1,029,900
Options, Exercised | shares	654,200
Options, Canceled/Forfeited | shares	27,800
Options, End of Year | shares	347,900
Options, Exercisable at End of Year | shares	347,900
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Options, Beginning of Year, Weighted Average Exercise Price | $ / shares	$ 37.93
Options, Exercised, Weighted Average Exercise Price | $ / shares	40.02
Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price | $ / shares	44.44
Options, End of Year, Weighted Average Exercise Price | $ / shares	33.49
Options, Exercisable at End of Year, Weighted Average Exercise Price | $ / shares	$ 33.49
Options, Outstanding at End of Year, Weighted Average Remaining Years Contractual Term	1 year 11 months 12 days
Options, Exercisable at End of Year, Weighted Average Remaining Years Contractual Term	1 year 11 months 12 days
Options, Outstanding at End of Year, Aggregate Intrinsic Value | $	$ 6,265,679
Options, Exercisable at End of Year, Aggregate Intrinsic Value | $	$ 6,265,679