Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding Table Footnote (Detail) - shares shares in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Class A Common Stock
Class of Stock [Line Items]
Common stock, shares issued (in shares)	13.8	0.7	15.4	2.1
Limited Partnership
Class of Stock [Line Items]
Number of units redeemed and cancelled for exchange (in shares)	14.6	0.6	16.3	2.1