Equity - Summary of Class A Common Stock Issuances (Parenthetical) (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	May 31, 2013	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Shares Issued	25,875,000		10,842,746	10,535,842	14,568,431
Class A Common Stock [Member]
Shares Issued			10,840,696	10,534,628	14,566,743
Manager [Member]
Management fees paid to Manager			$ 78.9	$ 76.9	$ 70.5
Manager [Member] | Class A Common Stock [Member]
Management fees paid to Manager		$ 19.3
Stock Issued During Period, Shares, Issued for Services		840,696	840,696
Class A Common Stock [Member] | Manager [Member]
Shares Issued			840,696
Weighted Average Price Per Share			$ 22.93
At The Market [Member] | Class A Common Stock [Member]
Shares Issued				1,900,000	7,700,000
Weighted Average Price Per Share				$ 34.63	$ 33.66