Summary of Authorized and Issued Shares (Detail) (USD $)	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Class of Stock [Line Items]
Preferred stock, par value	$ 0.01
Preferred stock, shares authorized	5,000,000	5,000,000	5,000,000
Preferred stock, shares issued	0	0	0
Common stock, par value	$ 0.01
Common stock, shares authorized	400,000,000	400,000,000	400,000,000
Common stock, shares issued	296,150,999	295,258,674	295,099,324