Stock Transactions and Unit Redemptions - Changes in Shares of Class A Common Stock Outstanding (Parenthetical) (Detail) - shares shares in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Limited Partnership [Member]
Class of Stock [Line Items]
Number of units redeemed and cancelled for exchange	11.5	7.6
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued	10.1	6.8