Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2011	Dec. 31, 2010
Treasury securities, shares	118,357	110,540
Common Stock [Member]
Common stock, shares outstanding	35,983,965	34,829,382
Common Class A [Member]
Common stock, shares authorized	175,000,000	175,000,000
Common stock, shares issued	36,102,322	34,939,922
Common stock, shares outstanding	35,983,965	34,829,382
Common stock, par value	0.01	0.01
Common Class B [Member]
Common stock, shares authorized	1	1
Common stock, shares issued	1	1
Common stock, shares outstanding	1	1
Common stock, par value	0.01	0.01