Convertible Senior Notes - Net proceeds (Details)			1 Months Ended								2 Months Ended	3 Months Ended					6 Months Ended		9 Months Ended		12 Months Ended
	Jul. 01, 2020 shares	May 18, 2020 USD ($) D $ / shares	Feb. 28, 2021 USD ($) shares	Jan. 31, 2021 USD ($) shares	Nov. 30, 2020 $ / shares shares	Aug. 31, 2020 $ / shares shares	May 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) shares	Mar. 31, 2019 $ / shares shares	Feb. 28, 2021 USD ($) shares	Dec. 31, 2020 USD ($) $ / shares shares	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Mar. 31, 2019 USD ($) $ / shares	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($)	May 29, 2020 USD ($)	Mar. 31, 2018 USD ($)
Convertible Senior Notes
Interest rate (as a percent)							3.50%
Net proceeds														$ 9,024,000			$ 27,678,000	$ 25,609,000	$ 47,568,000	$ 57,249,000	$ 65,259,000	$ 83,668,000	$ 76,175,000
Aggregate consideration																	90,238,000		90,238,000		90,238,000
Conversion of convertible notes to common stock																					62,553,000
Convertible senior notes												$ 85,600,000									85,600,000
Convertible senior note														$ 441,000			441,000		43,058,000	1,883,000	1,179,000	28,392,000
Proceeds from issuance of convertible senior notes, net																	205,100,000		205,098,000	39,052,000	205,098,000	39,052,000	$ 95,856,000
Gain (loss) on extinguishment of debt													$ 13,222,000		$ (518,000)	$ (518,000)	$ 13,222,000	$ (518,000)	$ 13,222,000	(518,000)	$ 17,686,000	$ (518,000)
Closing stock price on grant date | $ / shares					$ 22.25	$ 10.25		$ 2.75		$ 2.35						$ 2.35						$ 2.75
Common stock shares issued | shares					43,700,000	35,276,250		46,000,000		10,000,000		4,400,000									79,000,000.0
Subsequent event
Convertible Senior Notes
Common stock shares issued | shares			54,966,188
Level 3
Convertible Senior Notes
Fair value of convertible senior notes								$ 53,455,000														$ 53,455,000
3.75% Convertible Senior Notes
Convertible Senior Notes
Principal amount		$ 200,000,000.0					$ 212,463,000					$ 212,463,000									$ 212,463,000			$ 12,500,000
Interest rate (as a percent)		3.75%					3.75%					3.75%									3.75%			3.75%
Net proceeds							$ 189,219,000
Conversion of notes through common stock issuance | shares											3,000,000.0
Maturity principal amount		$ 1,000
Conversion rates for the notes (in shares)		198.6196
Conversion price, per share | $ / shares		$ 5.03
Trading days | D		20
Consecutive trading days | D		30
Conversion price (as a percent)		130.00%
Number of business days		5 days
Number of consecutive trading days		5 days
Principal amount (as a percent)		98.00%
Percentage of principal amount to be redeemed		100.00%
Carrying amount of the liability component		$ 75,200,000
Carrying amount of the equity component		$ 130,300,000										$ 130,249,000									$ 130,249,000
Effective interest rate (as a percent)		29.00%
Transaction costs for issuance		$ 7,000,000.0
Initial purchasers' discount		6,400,000					6,374,000
Other issuance costs		600,000					617,000
Transaction costs attributable to the liability component		2,600,000
Transaction costs attributable to the equity component		$ 4,400,000
Closing stock price on grant date | $ / shares												$ 33.91									$ 33.91
Fair value of convertible senior notes												$ 1,300,000,000									$ 1,300,000,000
Income tax benefit on equity component																					$ 29,800,000
3.75% Convertible Senior Notes | Subsequent event
Convertible Senior Notes
Interest rate (as a percent)			3.75%	3.75%							3.75%
Conversion of notes through common stock issuance | shares			3,000,000.0	3,000,000.0
Convertible senior note			$ 15,200,000	$ 15,200,000							$ 15,200,000
3.75% Convertible Senior Notes | Minimum
Convertible Senior Notes
Redemption notice days		1 day
3.75% Convertible Senior Notes | Maximum
Convertible Senior Notes
Redemption notice days		3 days
3.75% Convertible Senior Notes | Private placement
Convertible Senior Notes
Principal amount							$ 212,500,000
Interest rate (as a percent)							3.75%
7.5% Convertible Senior Note
Convertible Senior Notes
Principal amount									$ 40,000,000.0						$ 40,000,000.0					$ 40,000,000.0
Interest rate (as a percent)	7.50%								7.50%			7.50%			7.50%					7.50%	7.50%
Conversion of notes through common stock issuance | shares	16,000,000.0								16,000,000.0
Maturity principal amount									$ 48,000,000.0						$ 48,000,000.0					$ 48,000,000.0
Proceeds from issuance of convertible senior notes, net									$ 39,100,000
Percentage of principal amount to be redeemed									120.00%
5.5% Convertible Senior Notes
Convertible Senior Notes
Principal amount		$ 100,000,000						100,000,000				$ 160,000									$ 160,000	100,000,000			$ 100,000,000
Interest rate (as a percent)		5.50%					5.50%					5.50%									5.50%				5.50%
Repurchase of convertible senior notes | shares							9,400,000
Aggregate repurchase of debt							$ 128,900,000
Aggregate consideration							90,200,000
Conversion of notes through common stock issuance | shares												14,600,000
Debt component of debt exchange							35,500,000
Equity component of debt exchange							$ 93,400,000
Discount rate for fair value of liability							29.80%
Long-term debt							$ 48,700,000
Convertible senior notes												$ 33,500,000									$ 33,500,000
Maturity principal amount							66,300,000
Gain on early debt extinguishment							13,200,000
Carrying amount of the liability component							58,200,000
Carrying amount of the equity component							$ 37,700,000	$ 37,702,000														37,702,000
Effective interest rate (as a percent)							16.00%
Transaction costs for issuance							$ 4,100,000
Initial purchasers' discount							3,300,000
Other issuance costs							900,000															1,700,000
Gain (loss) on extinguishment of debt												$ 4,500,000
Transaction costs attributable to the liability component							2,400,000
Transaction costs attributable to the equity component							$ 1,700,000
Income tax benefit on equity component																						$ 9,200,000