Stockholders' Equity (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended						12 Months Ended
	Feb. 28, 2011	Aug. 31, 2009	May 31, 2009	Feb. 28, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Employee Stock Purchase Plan [Member] years	Dec. 31, 2010 Employee Stock Purchase Plan [Member]	May 31, 2011 Prior To Amendment [Member]	May 31, 2011 After Amendment [Member]
Stockholders' Equity [Line Items]
Common stock public offering of shares	11,500,000	12,650,000	1,178,163	5,740,000
Common stock public offering price per share	$ 15.50	$ 10.75	$ 9.72	$ 9.72
Net proceeds of common stock public offering	$ 168.1	$ 128.2	$ 11.5	$ 52.5
Common stock available for issuance					250,000,000	150,000,000			150,000,000	250,000,000
Common stock available for issuance in Employee Stock Purchase Plan							799,000
Percentage of purchase of shares in fair market value under Stock Purchase Plan							85.00%	85.00%
Number of years of offering period							2