Summary of Significant Accounting Policies (Net Income Per Share) (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted Average Number of Shares Outstanding, Basic	64,905	71,746	72,420
Incremental Weighted Average Shares Attributable to Dilutive Effect [Abstract]
Weighted Average Number of Shares Outstanding, Diluted	65,625	72,410	73,190
Employee Stock Option [Member]
Incremental Weighted Average Shares Attributable to Dilutive Effect [Abstract]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	1	13	148
Restricted Stock Units (RSUs) [Member]
Incremental Weighted Average Shares Attributable to Dilutive Effect [Abstract]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	719	651	622