Balance Sheet Parenthetical (Parenthetical) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Class A common shares [Member]
Class of Stock [Line Items]
Common stock, par value per share	$ 0.01	$ 0.01
Common stock, shares authorized	180,000,000	180,000,000
Common stock, shares issued	65,516,757	64,584,102
Common stock shares outstanding	52,824,345	51,934,137
Shares in treasury	12,692,412	12,649,965
Class B common shares [Member]
Class of Stock [Line Items]
Common stock, par value per share	$ 0.01	$ 0.01
Common stock, shares authorized	30,000,000	30,000,000
Common stock, shares issued	1,053,985	1,608,989
Common stock shares outstanding	1,053,985	1,608,989