Stock Based Compensation (Restricted Stock Units Activity) (Details) - Restricted Stock Units (RSUs) [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	322,196
Shares, Granted	212,158
Shares, Vested	(303,092)
Shares, Canceled	(17,363)
Shares, Outstanding at End of Year	213,899	322,196
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	$ 38.75
Shares, Granted, Weighted Average Grant Date Fair Value	44.33	$ 42.28	$ 39.65
Shares, Vested, Weighted Average Grant Date Fair Value	39.96
Shares, Canceled, Weighted Average Grant Date Fair Value	41.76
Shares, Outstanding at End of Year, Weighted Average Grant Date Fair Value	$ 42.32	$ 38.75
Shares, Outstanding at End of Year, Weighted Average Remaining Years Contractual Term	1 year
Shares, Outstanding at End of Year, Aggregate Intrinsic Value	$ 11,015,850