Convertible Notes (Details)				9 Months Ended	12 Months Ended
	Apr. 06, 2018 USD ($) day	Apr. 04, 2018 USD ($)	Mar. 27, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2012 $ / shares
Debt Instrument [Line Items]
Proceeds from issuance of convertible note					$ 171,500,000	$ 0	$ 0
Over-allotment option, term			13 days
Over-allotment option			$ 22,500,000.0
Additional proceeds	$ 21,500,000
Conversion ratio			0.0323428
Initial conversion price (usd per share) | $ / shares								$ 1.03
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					48,400,000
Debt issuance costs, net					3,000,000
Interest expense					3,300,000
Amortization of debt issuance costs					399,000
Amortization of the debt discount					$ 6,500,000
Interest paid				$ 2,000,000
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]
Proceeds from issuance of convertible note		$ 171,500,000
Aggregate principal amount			$ 150,000,000
Interest rate			2.50%
Conversion multiple	1,000
Initial conversion price (usd per share) | $ / shares			$ 30.92
Long-term debt	116,600,000				$ 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					$ 120,100,000
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 | Contributed Capital
Debt Instrument [Line Items]
Debt issuance costs	$ 1,600,000