Stockholders' Equity (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Stockholders' Equity
Authorized capital stock (in shares)	260,000,000
Authorized shares, par value (in dollars per share)	$ 0.01
Common stock, shares authorized	160,000,000	160,000,000
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Excess stock, shares authorized	75,000,000
Excess stock, par value (in dollars per share)	$ 0.01
Preferred stock, shares authorized	25,000,000
Common stock, shares issued	95,201,000	94,896,000
Common stock, shares outstanding net of treasury shares	91,555,082
Excess stock, shares issued	0
Excess stock, shares outstanding	0