Stock-Based Compensation Plans (Details Textual) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Additional Stock-based Compensation Plans (Textual) [Abstract]
Cash received from option exercises and employee stock purchase plan purchases			$ 3,000,000	$ 3,100,000
Tax benefit realized for the tax deductions			1,100,000	1,600,000
Share-based Compensation	$ 3,600,000	$ 3,000,000	$ 6,960,000	$ 5,915,000
Long Term Incentive Plan [Member]
Stock-based Compensation Plans (Textual) [Abstract]
Maximum number of shares available for issuance under the plan	7,500,000		7,500,000
Expiry date for stock options grant	Mar. 08, 2017
Shares available for granted under the incentive plan	817,174		817,174
Employee Stock Purchase Plan [Member]
Stock-based Compensation Plans (Textual) [Abstract]
Shares authorized for issuance	3,500,000		3,500,000
Employee stock purchase price in percentage of fair market value	85.00%
Shares remaining in reserve for future issuance	665,734		665,734
Shares issued to employee under purchase plan	58,525	56,149
Weighted average fair value of employee stock purchase rights issued	$ 13.40	$ 11.68