Stockholders' Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Class of stock
Common stock, shares authorized to issue	200,000,000	200,000,000
Common stock, shares issued	100,042,976	99,393,780
Common stock, shares outstanding	100,042,976	99,393,780
Net proceeds from transactions	$ 3,567	$ 4,356	$ 13,156
Preferred Stock
Class of stock
Preferred stock, shares authorized to issue	15,000,000
Series A preferred stock
Class of stock
Dividend rate (as a percent)	5.00%
Series B
Class of stock
Dividend rate (as a percent)	5.00%
Common stock
Class of stock
Common stock, shares authorized to issue	200,000,000	200,000,000
Common stock, shares issued	100,042,976	99,393,780
Common stock, shares outstanding	100,042,976	99,393,780
Common shares reserved for future issuance	21,833,053
Options exercises and employee stock purchase plan issuances (in shares)	646,000	475,000
Net proceeds from transactions	$ 3,600	$ 4,400