Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding (Parenthetical) (Detail) - shares shares in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued	0.7	2.3	2.1	2.4
Limited Partnership [Member]
Class of Stock [Line Items]
Number of units redeemed and cancelled for exchange	0.6	2.5	2.1	2.6