The number of shares issued during the period upon the conversion of units. An example of a convertible unit is an umbrella partnership real estate investment trust unit (UPREIT unit).

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Name:	us-gaap_StockIssuedDuringPeriodSharesConversionOfUnits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding (Parenthetical) (Detail) - shares shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued	2.8	0.0	5.2	0.0
Limited Partnership [Member]
Class of Stock [Line Items]
Number of units redeemed and cancelled for exchange	3.2		5.8