Assets and Liabilities Measured at Fair Value on a Recurring Basis (Parenthetical) (Detail) (Fair Value, Measurements, Recurring, USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 11,372,081	[1]	$ 8,546,528	[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	5,945	[1]	411	[2]
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	799,962	[1]	547,608	[2]
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	10,566,174	[1]	7,998,509	[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	1,200
Noncontrolling Interest | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	647,500		423,500
Noncontrolling Interest | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets			$ 8,100

[1] 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.

[2] Included in Level 2 and Level 3 assets are $8.1 million and $423.5 million, respectively, attributable to noncontrolling interests calculated based on the ownership percentages of the noncontrolling interests.