Assets and Liabilities Measured at Fair Value on a Recurring Basis (Parenthetical) (Detail) (Fair Value, Measurements, Recurring, USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 11,924,755	[1]	$ 11,372,081	[2]
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	11,724	[1]	5,945	[2]
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	841,722	[1]	799,962	[2]
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	11,071,309	[1]	10,566,174	[2]
Noncontrolling Interest | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	3,400		1,200
Noncontrolling Interest | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	689,600		647,500
Noncontrolling Interest | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 1,500

[1] Included in Level 1, Level 2, and Level 3 assets are $3.4 million, $1.5 million, and $689.6 million, respectively, attributable to noncontrolling interests calculated based on the ownership percentages of the noncontrolling interests.

[2] Included in Level 1 and Level 3 assets are $1.2 million and $647.5 million, respectively, attributable to noncontrolling interests calculated based on the ownership percentages of the noncontrolling interests.