Balance Sheet Parenthetical (Parenthetical) (USD $)	Sep. 30, 2013	Dec. 31, 2012
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	64,490,314	57,882,442
Common stock shares outstanding	51,843,149	45,359,083
Shares in treasury	12,647,165	12,523,359
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	1,636,089	1,728,952
Common stock shares outstanding	1,636,089	1,728,952