Commitments and Contingencies - Additional Information (Details)	6 Months Ended
	Jun. 30, 2019 USD ($) employee	Jun. 30, 2018 USD ($) employee	Dec. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Feb. 28, 2017 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2012 USD ($) employee
Operating Leased Assets [Line Items]
Operating leases, rent expense	$ 3,800,000	$ 2,900,000
Sale leaseback rent expense	0	0
Grants receivable							$ 16,500,000
Number of employees to be hired per incentive grant agreement | employee							900
Minimum cumulative employee compensation, recapture period one				$ 108,000,000
Minimum cumulative employee compensation, recapture period three							$ 324,000,000
Cumulative compensation expense incurred	105,600,000	$ 80,400,000
Proceeds from government grants	12,000,000
Grant agreement, maximum possible repayment amount, recapture period two							5,000,000
Grant agreement, maximum possible repayment amount, recapture period three							$ 2,500,000
Grant agreement, recapture provision repayments	1,500,000
Delaware grant obligation	$ 10,469,000		$ 10,469,000
SGIP, percentage of incentive issued in the first year	50.00%
SGIP, subsequent payment period	5 years
Delaware
Operating Leased Assets [Line Items]
Number of full time employees | employee	317	328
PPA V
Operating Leased Assets [Line Items]
PPA expenses	$ 3,500,000	$ 900,000
Term loan | PPA V | Term Loan due December 2021, Non-Recourse
Operating Leased Assets [Line Items]
Debt face amount						$ 131,200,000
Letters of Credit | PPA V
Operating Leased Assets [Line Items]
Maximum borrowing capacity					$ 6,200,000.0	$ 6,400,000.0
Amount outstanding	$ 5,000,000		$ 5,000,000