Stockholders' Equity (Detail) - USD ($)	9 Months Ended	12 Months Ended	36 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	May 01, 2021	Jun. 21, 2021	Mar. 16, 2020	May 01, 2018
Shelf Registration
Class of Stock [Line Items]
Common stock shares issued for cash (in shares)	0	0
Common Class A | Stock Repurchase Program
Class of Stock [Line Items]
Stock repurchase program, authorized amount					$ 250,000,000
Stock repurchased during period	$ 0
Sales Agreement | Common Class A
Class of Stock [Line Items]
Common stock, shares sold (in shares)			842,412
2021 Sales Agreement | Common Class A
Class of Stock [Line Items]
Common stock, shares sold (in shares)	0
Common stock, shares available to be sold	$ 400,000,000
Maximum | Sales Agreement | Common Class A
Class of Stock [Line Items]
Aggregate offering price of common stock issuable						$ 400,000,000
Maximum | 2021 Sales Agreement | Common Class A
Class of Stock [Line Items]
Aggregate offering price of common stock issuable				$ 400,000