Impairment Charges and Impairment of Joint Venture Investments - Summary of Significant Unobservable Inputs (Detail) (Fair Value Measurements [Member], USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Impairment of Consolidated Assets [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Unobservable Input	Price Per Square Foot	Price Per Square Foot
Impairment of Joint Venture Investments, Income Capitalization Approach [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Unobservable Input	Terminal Capitalization Rate	Terminal Capitalization Rate
Income Capitalization Approach [Member] \| Impairment of Consolidated Assets [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value	$ 74.3	$ 44.1
Valuation Technique	Income Capitalization Approach(B)	Income Capitalization Approach (B)
Unobservable Input	Market Capitalization Rate	Market Capitalization Rate
Income Capitalization Approach [Member] \| Impairment of Joint Venture Investments, Income Capitalization Approach [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value	0	4.7
Valuation Technique	Income Capitalization Approach	Income Capitalization Approach
Unobservable Input	Market Capitalization Rate	Market Capitalization Rate
Indicative Bid/Contracted Price [Member] \| Impairment of Consolidated Assets [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value	87.1	136.6
Valuation Technique	Indicative Bid(A) / Contracted Price	Indicative Bid (A) / Contracted Price
Unobservable Input	Indicative Bid(A) / Contracted Price	Indicative Bid (A) / Contracted Price
Discounted Cash Flow [Member] \| Impairment of Joint Venture Investments, Income Capitalization Approach [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Valuation Technique	Discounted Cash Flow	Discounted Cash Flow
Unobservable Input	Discount Rate	Discount Rate
Discounted Cash Flow [Member] \| Deconsolidated Joint Venture Investment, Discounted Cash Flow [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Fair Value	$ 0	$ 56.1
Valuation Technique	Discounted Cash Flow	Discounted Cash Flow
Unobservable Input	Discount Rate	Discount Rate
Level 3 [Member] \| Income Capitalization Approach [Member] \| Impairment of Joint Venture Investments, Income Capitalization Approach [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Range	0.00%	8.00%
Level 3 [Member] \| Discounted Cash Flow [Member] \| Impairment of Joint Venture Investments, Income Capitalization Approach [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Range	0.00%	11.00%
Minimum [Member] \| Level 3 [Member] \| Impairment of Joint Venture Investments, Income Capitalization Approach [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Range	0.00%	5.50%
Minimum [Member] \| Level 3 [Member] \| Income Capitalization Approach [Member] \| Impairment of Consolidated Assets [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Range	7.70%	8.00%
Range (per square foot)	12	15
Minimum [Member] \| Level 3 [Member] \| Discounted Cash Flow [Member] \| Deconsolidated Joint Venture Investment, Discounted Cash Flow [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Range	0.00%	8.00%
Maximum [Member] \| Level 3 [Member] \| Impairment of Joint Venture Investments, Income Capitalization Approach [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Range	0.00%	8.50%
Maximum [Member] \| Level 3 [Member] \| Income Capitalization Approach [Member] \| Impairment of Consolidated Assets [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Range	10.00%	12.00%
Range (per square foot)	117	47
Maximum [Member] \| Level 3 [Member] \| Discounted Cash Flow [Member] \| Deconsolidated Joint Venture Investment, Discounted Cash Flow [Member]
Fair Value Assets And Liabilities Measured On Nonrecurring Basis [Line Items]
Range	0.00%	15.00%