Real Estate and Accumulated Depreciation (Parenthetical) (Detail) (USD $)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Real Estate and Accumulated Depreciation [Line Items]
Tax cost basis of assets	$ 8,700,000,000	$ 8,500,000,000	$ 8,600,000,000
Real estate land under development	254,600,000
Real estate construction in progress	220,500,000
The amount of tax-exempt certificates excluded from encumbrances	15,700,000
Fair market value of debt adjustments excluded from encumbrances	$ 11,500,000