Nature of Operations (Details) $ / shares in Units, $ in Thousands		1 Months Ended			3 Months Ended		12 Months Ended
	Jul. 21, 2017 USD ($)	Apr. 30, 2019 USD ($)	Mar. 31, 2019 USD ($) $ / shares shares	Nov. 30, 2018 USD ($) $ / shares shares	Mar. 31, 2019 USD ($) item $ / shares shares	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Liquidity
Net loss attributable to common shareholders					$ 33,990	$ 19,848	$ 78,200	$ 130,200	$ 57,600
Accumulated deficit			$ 1,294,241		1,294,241		1,260,290
Net cash used in operating activities					36,263	24,028
Net loss attributable to the Company					(33,938)	(19,835)
Net outflows from fluctuations in working capital and other assets and liabilities					16,900
Non-cash charges					14,500
Cash and cash equivalents			39,336		39,336		38,602
Net working capital			77,700		77,700		$ 9,200
Net cash used in investing activities					(2,274)	(4,303)
Net cash provided by financing activities					37,650	49,199
Sale of common stock			23,500		23,498
Proceeds from borrowing of long-term debt					84,761
Repayments of Long-term Debt					17,153	$ 4,649
Remaining lease payments			77,500		77,500
Security on lease			78,200		78,200
Letter of credit			$ 35,100		$ 35,100
Sale/leaseback transactions under Wells Fargo MLA | item					0
Repayments of finance obligations					$ 53,534
Number of common stock sold | shares			10,000,000		10,000,000
Par value, common stock | $ / shares			$ 0.01		$ 0.01		$ 0.01
Share price (in dollars per share) | $ / shares			$ 2.35		$ 2.35
Wells Fargo
Liquidity
Remaining lease payments			$ 64,900		$ 64,900
Letter of credit							$ 20,100
Series E Redeemable Convertible Preferred Stock
Liquidity
Shares issued (in shares) | shares				35,000
Redeemable stock issued, stated value (in dollars per share) | $ / shares			$ 0.01	$ 0.01	$ 0.01		$ 0.01
Aggregate proceeds from issuance of preferred stock				$ 30,900
Loan and security agreement | Generate Lending, LLC
Liquidity
Repayments of finance obligations	$ 17,600				$ 17,600
Termination of certain equipment leases	$ 50,300				50,300
Loan and security agreement | Generate Lending, LLC | Secured term loan facility
Liquidity
Proceeds from borrowing of long-term debt		$ 15,000			$ 85,000
Loan and security agreement | Generate Lending, LLC | Secured term loan facility | Subsequent event
Liquidity
Proceeds from borrowing of long-term debt		$ 15,000