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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued	0.5	2.8	2.6	5.2
Limited Partnership [Member]
Class of Stock [Line Items]
Number of units redeemed and cancelled for exchange	0.4	3.2	2.5	5.8