Stockholders' Equity - Common Stock Outstanding, Including Unvested Restricted Shares of Common Stock (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Class of Stock [Line Items]
Common Stock, Shares, Outstanding, Beginning Balance	86,807,402
Issuance of Restricted Stock Shares	565,137
Common Stock, Shares, Outstanding, Ending Balance	98,767,913
Common Class A [Member]
Class of Stock [Line Items]
Common Stock, Shares, Outstanding, Beginning Balance	86,807,402	68,841,296	61,845,214
Issuance of Common Stock, Including Vesting of Restricted Stock Units	11,085,905	17,646,586	6,518,736
Issuance of Restricted Stock Shares	565,137	292,339	573,198
Repurchase and Retirement of Restricted Stock Shares	(225,557)	(98,603)	(123,438)
Conversion of Operating Partnership Units	535,026	125,784	27,586
Common Stock, Shares, Outstanding, Ending Balance	98,767,913	86,807,402	68,841,296