Convertible Notes - Narrative (Details)			6 Months Ended		12 Months Ended
	Apr. 04, 2018 USD ($) day	Mar. 27, 2018 USD ($) $ / shares	Jun. 30, 2021 USD ($) shares	Jun. 30, 2020 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2020 USD ($)
Debt Instrument [Line Items]
Proceeds from debt			$ 0	$ 4,791,000
Outstanding principal			171,500,000			$ 171,500,000
Unamortized debt issuance			$ (1,410,000)			$ (1,765,000)
Net proceeds from notes to fund Prepaid Forward	$ 45,100,000
Aggregate number of shares (shares) | shares			1,858,500
Unsecured obligations
Debt Instrument [Line Items]
Aggregate principal amount			$ 5,300,000
Unsecured obligations | 2.50% Convertible notes due 2023
Debt Instrument [Line Items]
Aggregate principal amount	171,500,000	$ 150,000,000.0
Interest rate		2.50%	2.50%
Over-allotment option, term		13 days
Over-allotment option		$ 22,500,000
Additional proceeds	$ 21,500,000
Proceeds from debt					$ 171,500,000
Conversion ratio	0.0323428	0.0323428
Initial conversion price (usd per share) | $ / shares		$ 30.92
Outstanding principal	$ 116,600,000
Equity component	(54,900,000)
Unamortized debt issuance	$ (5,000,000.0)
Contractual term	5 years
Effective interest rate	11.52%
Stock price conversion threshold, percentage	130.00%
Consecutive trading days | day	20
Threshold trading days | day	30
Trading price threshold, percentage	98.00%
Repurchase principal balance, percent	100.00%
Unsecured obligations | 2.50% Convertible notes due 2023 | Long-term debt
Debt Instrument [Line Items]
Unamortized debt issuance	$ (3,400,000)
Unsecured obligations | 2.50% Convertible notes due 2023 | Contributed capital
Debt Instrument [Line Items]
Unamortized debt issuance	$ (1,600,000)