Stock-Based Compensation (Summary of Restricted Stock Activity and Unvested Shares) (Details) (USD $)	6 Months Ended
	Jun. 30, 2013
Stock-Based Compensation [Abstract]
Restricted Stock, Unvested shares at December 31, 2012 Number of Shares	1,117,515
Restricted Stock, Granted, Number of Shares	13,540
Restricted Stock, Vested, Number of Shares	(26,044)
Restricted Stock, Forfeited, Number of Shares	(41,501)
Restricted Stock, Unvested shares at June 30, 2013, Number of Shares	1,063,510
Restricted Stock, Unvested shares at December 31, 2012 Weighted Average Grant Date Fair Value	$ 35.64
Restricted Stock, Granted, Weighted Average Grant Date Fair Value	$ 36.60
Restricted Stock, Vested, Weighted Average Grant Date Fair Value	$ 36.17
Restricted Stock, Forfeited, Weighted Average Grant Date Fair Value	$ 35.88
Restricted Stock, Unvested shares at June 30, 2013, Weighted Average Grant Date Fair Value	$ 35.63