Impairment Charges and Reserves - Summary of Significant Unobservable Inputs (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Consolidated Assets, Fair Value	$ 452.7
Preferred Equity Interests, Fair Value	199.3
Fair Value Measurements [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Consolidated Assets, Fair Value	452.7
Preferred Equity Interests, Fair Value	199.3
Level 3 [Member] | Fair Value Measurements [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Consolidated Assets, Fair Value	452.7
Preferred Equity Interests, Fair Value	199.3
Impairment of Consolidated Assets [Member] | Level 3 [Member] | Indicative Bid [Member] | Fair Value Measurements [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Consolidated Assets, Fair Value	351.2
Impairment of Consolidated Assets [Member] | Level 3 [Member] | Income Capitalization Approach [Member] | Fair Value Measurements [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Consolidated Assets, Fair Value	$ 694.1
Impairment of Consolidated Assets [Member] | Level 3 [Member] | Income Capitalization Approach [Member] | Fair Value Measurements [Member] | Minimum [Member] | Measurement Input Cap Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market capitalization rate	7.40%
Impairment of Consolidated Assets [Member] | Level 3 [Member] | Income Capitalization Approach [Member] | Fair Value Measurements [Member] | Maximum [Member] | Measurement Input Cap Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market capitalization rate	9.30%
Impairment of Consolidated Assets [Member] | Level 3 [Member] | Discounted Cash Flow [Member] | Fair Value Measurements [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Consolidated Assets, Fair Value	$ 32.0
Terminal capitalization rate	10.50%
Impairment of Consolidated Assets [Member] | Level 3 [Member] | Discounted Cash Flow [Member] | Fair Value Measurements [Member] | Measurement Input Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market capitalization rate	9.50%
Reserve of Preferred Equity Interests [Member] | Level 3 [Member] | Discounted Cash Flow [Member] | Fair Value Measurements [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred Equity Interests, Fair Value	$ 199.3
NOI growth rate	1.00%
Reserve of Preferred Equity Interests [Member] | Level 3 [Member] | Discounted Cash Flow [Member] | Fair Value Measurements [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Terminal capitalization rate	7.70%
Reserve of Preferred Equity Interests [Member] | Level 3 [Member] | Discounted Cash Flow [Member] | Fair Value Measurements [Member] | Minimum [Member] | Measurement Input Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market capitalization rate	8.20%
Reserve of Preferred Equity Interests [Member] | Level 3 [Member] | Discounted Cash Flow [Member] | Fair Value Measurements [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Terminal capitalization rate	8.50%
Reserve of Preferred Equity Interests [Member] | Level 3 [Member] | Discounted Cash Flow [Member] | Fair Value Measurements [Member] | Maximum [Member] | Measurement Input Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market capitalization rate	8.90%