Convertible Notes (Details)			3 Months Ended	9 Months Ended
	Apr. 06, 2018 USD ($) day shares	Mar. 27, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
Debt Instrument [Line Items]
Over-allotment option, term		13 days
Over-allotment option		$ 22,500,000.0
Additional proceeds	$ 21,500,000
Conversion ratio		0.0323428
Stock price conversion threshold, percentage	130.00%
Consecutive trading days | day	20
Threshold trading days	30
Trading price threshold, percentage	98.00%
Repurchase principal balance, percent	100.00%
Unamortized debt discount			$ 50,600,000	$ 50,600,000
Debt issuance costs, net			3,100,000	3,100,000
Interest expense			1,100,000	2,200,000
Amortization of debt issuance costs			131,000	264,000
Amortization of the debt discount			2,100,000	4,300,000
Interest paid				2,001,000
Net proceeds from notes to fund Prepaid Forward	$ 45,100,000
Aggregate number of 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
Interest rate		2.50%
Proceeds received	171,500,000
Conversion multiple	1,000
Initial conversion price (usd per share) | $ / shares		$ 30.92
Long-term debt	116,600,000		171,500,000	171,500,000
Equity component	54,900,000
Debt issuance costs	$ 5,000,000
Contractual term	5 years
Effective interest rate	11.52%
Net carrying amount			$ 117,800,000	$ 117,800,000
Unsecured obligations | 2.50% Convertible notes due 2023 | Long-term debt
Debt Instrument [Line Items]
Debt issuance costs	$ 3,400,000
Unsecured obligations | 2.50% Convertible notes due 2023 | Additional paid-in capital
Debt Instrument [Line Items]
Debt issuance costs	$ 1,600,000