Stockholders' Equity - Registration Rights Agreement (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 11, 2019	Dec. 31, 2018	Dec. 31, 2016	Jun. 30, 2016
Debt Instrument [Line Items]
Maximum amount of repurchase rights agreement		$ 50
Effectiveness penalty percentage		0.25%
Purchase price allocation percentage		0.75%
5% Convertible Senior Notes due 2021
Debt Instrument [Line Items]
Debt instrument, interest rate		5.00%		5.00%
5% Convertible Senior Notes due 2021 | Convertible Debt
Debt Instrument [Line Items]
Interest rate on the 90th day following registration statement		0.25%
Interest rate on the 91st day following the registration statement		0.50%
Registration statement liquidation damages limit		0.50%
Debt instrument, interest rate		5.00%	5.00%
2.5% Convertible Senior Note | Convertible Debt
Debt Instrument [Line Items]
Registration statement liquidation damages limit		5.00%
Debt instrument, interest rate		2.50%
Liquidation damages percent		1.00%
Subsequent Event | Washington Refinery Acquisition
Debt Instrument [Line Items]
Issuance of common stock in connection with acquisition (in shares)	2,363,776