Debt - Additional Information (Detail) $ / shares in Units, shares in Millions, $ in Millions			3 Months Ended					6 Months Ended	9 Months Ended
	Dec. 17, 2020 USD ($) d $ / shares	Jun. 14, 2019 USD ($) d $ / shares	Sep. 30, 2021 USD ($) $ / shares shares	Mar. 31, 2021	Dec. 31, 2020 USD ($)	Sep. 30, 2020 shares	Jun. 30, 2020	Dec. 31, 2020 USD ($)	Sep. 30, 2021 USD ($) $ / shares shares	Sep. 30, 2020 shares
Line Of Credit Facility [Line Items]
Aggregate principal amount			$ 1,564.5		$ 1,564.5			$ 1,564.5	$ 1,564.5
Debt instrument carrying amount of equity component conversion option			$ 278.6		$ 278.6			$ 278.6	$ 278.6
Anti-dilutive securities underlying conversion option | shares			103.5			109.6			111.1	83.9
Bank of America, N.A. [Member] | Credit Agreement [Member]
Line Of Credit Facility [Line Items]
Credit facility expiration period									3 years
Initial aggregate principal borrowing capacity amount			$ 425.0						$ 425.0
Debt instrument interest rate, description									Under the 2020 Credit Agreement, at the Company’s option, revolving loans accrue interest at a per annum rate based on either (i) the base rate plus a margin ranging from 0.50% to 1.00%, determined based on the Company’s consolidated leverage ratio for the four most recent fiscal quarters (the “Consolidated Leverage Ratio”) or (ii) the LIBOR rate (for interest periods of one, two, three or six months) plus a margin ranging from 1.50% to 2.00%, determined based on the Company’s Consolidated Leverage Ratio. The base rate is defined as the highest of (i) the federal funds rate, plus 0.50%, (ii) Bank of America, N.A.’s prime rate and (iii) the LIBOR rate for a one-month interest period plus 1.00%.
Debt instrument commitment fee, description									The Company is also obligated to pay an ongoing commitment fee on undrawn amounts at a rate ranging from 0.25% to 0.35%, determined based on the Company’s Consolidated Leverage Ratio.
Line of credit facility amount outstanding									$ 0.0
Bank of America, N.A. [Member] | Credit Agreement [Member] | Minimum [Member]
Line Of Credit Facility [Line Items]
Payment for commitment fee, percentage									0.25%
Bank of America, N.A. [Member] | Credit Agreement [Member] | Maximum [Member]
Line Of Credit Facility [Line Items]
Payment for commitment fee, percentage									0.35%
Bank of America, N.A. [Member] | Credit Agreement [Member] | Base Rate [Member] | Minimum [Member]
Line Of Credit Facility [Line Items]
Debt instrument interest rate				0.50%	0.50%	0.50%	0.50%
Bank of America, N.A. [Member] | Credit Agreement [Member] | Base Rate [Member] | Maximum [Member]
Line Of Credit Facility [Line Items]
Debt instrument interest rate				1.00%	1.00%	1.00%	1.00%
Bank of America, N.A. [Member] | Credit Agreement [Member] | LIBOR Rate [Member]
Line Of Credit Facility [Line Items]
Debt instrument interest rate									1.00%
Bank of America, N.A. [Member] | Credit Agreement [Member] | LIBOR Rate [Member] | Minimum [Member]
Line Of Credit Facility [Line Items]
Debt instrument interest rate								1.50%
Bank of America, N.A. [Member] | Credit Agreement [Member] | LIBOR Rate [Member] | Maximum [Member]
Line Of Credit Facility [Line Items]
Debt instrument interest rate								2.00%
Bank of America, N.A. [Member] | Credit Agreement [Member] | Federal Funds Rate [Member]
Line Of Credit Facility [Line Items]
Debt instrument interest rate									0.50%
Common Class A [Member] | Convertible Senior Notes And Capped Call Transactions [Member]
Line Of Credit Facility [Line Items]
Anti-dilutive securities underlying conversion option | shares									83.1
Common Class A [Member] | Convertible Senior Notes Due 2026 [Member]
Line Of Credit Facility [Line Items]
Anti-dilutive securities underlying conversion option | shares									66.9
Convertible Senior Notes Due 2026 [Member]
Line Of Credit Facility [Line Items]
Aggregate principal amount	$ 874.5		874.5		$ 874.5			$ 874.5	$ 874.5
Debt instrument, interest rate	0.00%
Additional aggregate principal amount	$ 112.5
Proceeds from the issuance of notes	$ 856.8
Debt instrument maturity date	Dec. 15, 2026
Debt instrument payment terms									The 2026 Notes and 2024 Notes mature on December 15, 2026 and June 1, 2024, respectively, unless earlier converted, redeemed or repurchased in accordance with their terms respectively prior to the maturity date. The 2026 Notes do not bear regular interest, and the principal amount does not accrete, while interest is payable semiannually on the 2024 Notes in arrears on June 1 and December 1 of each year.
Debt repurchase price percentage	100.00%
Debt instrument initial carrying amount	$ 707.4
Interest rate used to calculate the present value of the cash flows	3.50%
Debt instrument carrying amount of equity component conversion option	$ 167.1		$ 163.7		163.7			163.7	$ 163.7
Contractual term	6 years
Transaction costs attributable to the liability component	$ 14.3
Transaction costs attributable to the equity component	$ 3.4
Convertible Senior Notes Due 2026 [Member] | Capped Call Transactions [Member]
Line Of Credit Facility [Line Items]
Initial strike price | $ / shares			13.07						13.07
Initial cap price | $ / shares			17.42						17.42
Cost incurred for Capped Calls									$ 63.0
Convertible Senior Notes Due 2026 [Member] | Fair Value, Inputs, Level 2 [Member]
Line Of Credit Facility [Line Items]
Debt instrument estimated fair value			$ 849.1						$ 849.1
Convertible Senior Notes Due 2026 [Member] | Common Class A [Member]
Line Of Credit Facility [Line Items]
Initial conversion rate of common stock per $1,000 principal amount	76.5404
Initial conversion price per share of common stock | $ / shares	$ 13.07
Debt instrument threshold trading days to trigger conversion feature | d	20
Debt instrument threshold consecutive trading days to trigger conversion feature | d	30
Debt instrument, minimum percentage of common stock price to determine eligibility of conversion	130.00%
Consecutive business trading period when trading price meets required criteria as a condition for conversion of debt	5 days
Trading price percentage of product of last reported sales price as a condition for conversion of debt	98.00%
Debt instrument redemption start date	Dec. 20, 2023
Debt instrument redemption price percentage	100.00%
Convertible Senior Notes Due 2026 [Member] | Common Class A [Member] | Capped Call Transactions [Member]
Line Of Credit Facility [Line Items]
Capped calls economic dilutive potential common stock shares | shares									66.9
Convertible Senior Notes Due 2024 [Member]
Line Of Credit Facility [Line Items]
Aggregate principal amount		$ 690.0	690.0		690.0			690.0	$ 690.0
Debt instrument, interest rate		0.25%
Additional aggregate principal amount		$ 90.0
Proceeds from the issuance of notes		$ 672.2
Debt instrument maturity date		Jun. 01, 2024
Debt instrument payment terms									The 2026 Notes and 2024 Notes mature on December 15, 2026 and June 1, 2024, respectively, unless earlier converted, redeemed or repurchased in accordance with their terms respectively prior to the maturity date. The 2026 Notes do not bear regular interest, and the principal amount does not accrete, while interest is payable semiannually on the 2024 Notes in arrears on June 1 and December 1 of each year.
Debt repurchase price percentage		100.00%
Debt instrument initial carrying amount		$ 572.0
Interest rate used to calculate the present value of the cash flows		4.10%
Debt instrument carrying amount of equity component conversion option		$ 118.0	$ 114.9		$ 114.9			$ 114.9	$ 114.9
Contractual term		5 years
Transaction costs attributable to the liability component		$ 14.8
Transaction costs attributable to the equity component		$ 3.1
Convertible Senior Notes Due 2024 [Member] | Capped Call Transactions [Member]
Line Of Credit Facility [Line Items]
Initial strike price | $ / shares			8.31						8.31
Initial cap price | $ / shares			12.54						12.54
Cost incurred for Capped Calls									$ 73.8
Convertible Senior Notes Due 2024 [Member] | Fair Value, Inputs, Level 2 [Member]
Line Of Credit Facility [Line Items]
Debt instrument estimated fair value			$ 788.0						$ 788.0
Convertible Senior Notes Due 2024 [Member] | Common Class A [Member]
Line Of Credit Facility [Line Items]
Initial conversion rate of common stock per $1,000 principal amount		120.3695
Initial conversion price per share of common stock | $ / shares		$ 8.31
Debt instrument threshold trading days to trigger conversion feature | d		20
Debt instrument threshold consecutive trading days to trigger conversion feature | d		30
Debt instrument, minimum percentage of common stock price to determine eligibility of conversion		130.00%
Consecutive business trading period when trading price meets required criteria as a condition for conversion of debt		5 days
Trading price percentage of product of last reported sales price as a condition for conversion of debt		98.00%
Debt instrument redemption start date		Jun. 05, 2022
Debt instrument redemption price percentage		100.00%
Convertible Senior Notes Due 2024 [Member] | Common Class A [Member] | Capped Call Transactions [Member]
Line Of Credit Facility [Line Items]
Capped calls economic dilutive potential common stock shares | shares									83.1