Stock Based Compensation (Performance Units Information) (Details) - Performance Units [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period for recognizing unrecognized compensation cost	1 year
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Shares, Outstanding at Beginning of Year	393,812
Shares, Granted	382,830
Shares, Vested	(402,451)
Shares, Canceled	(41,730)
Shares, Outstanding at End of Year	332,461	393,812
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Shares, Outstanding at Beginning of Year, Weighted Average Grant Date Fair Value	$ 44.20
Shares, Granted, Weighted Average Grant Date Fair Value	45.02	$ 45.97	$ 41.32
Shares, Vested, Weighted Average Grant Date Fair Value	44.03
Shares, Cancelled, Weighted Average Grant Date Fair Value	44.69
Shares, Outstanding at End of Year, Weighted Average Grant Date Fair Value	$ 45.29	$ 44.20
Shares, Outstanding at End of Year, Weighted Average Remaining Years Contractual Term	1 year 8 months 12 days
Shares, Outstanding at End of Year, Aggregate Intrinsic Value	$ 17,121,742