Convertible Notes - Narrative (Details)			3 Months Ended		6 Months Ended
	Apr. 04, 2018 USD ($) day shares	Mar. 27, 2018 USD ($) $ / shares	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Over-allotment option, term		13 days
Over-allotment option		$ 22,500,000
Additional proceeds	$ 21,500,000
Conversion ratio		0.0323428
Outstanding principal			$ 171,500,000		$ 171,500,000		$ 171,500,000
Unamortized debt issuance			2,714,000		2,714,000		$ 2,996,000
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%
Interest expense			1,100,000	$ 1,000,000.0	2,100,000	$ 1,100,000
Amortization of debt issuance costs			100,000	100,000	300,000	100,000
Amortization of the debt discount			$ 2,300,000	$ 2,000,000.0	$ 4,600,000	$ 2,100,000
Net proceeds from notes to fund Prepaid Forward	$ 45,100,000
Aggregate number of shares (shares) | shares	1,858,500
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%
Proceeds received	171,500,000
Conversion multiple	1,000
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%
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