Commitments and Contingencies - Additional Information (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) employee	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) employee	Feb. 28, 2017 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2012 USD ($) employee
Operating Leased Assets [Line Items]
Operating leases, rent expense	$ 2,000,000		$ 1,400,000
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	99,300,000		92,000,000
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
Self-Generation Incentive Program, percentage of incentive issued in the first year	50.00%
Self-Generation Incentive Program, subsequent payment period	5 years
Delaware
Operating Leased Assets [Line Items]
Number of full time employees | employee	334		338
PPA Company V
Operating Leased Assets [Line Items]
PPA expenses	$ 3,500,000	$ 900,000
Term loan | PPA Company V | Term Loan due December 2021, Non-Recourse
Operating Leased Assets [Line Items]
Debt face amount					$ 131,200,000
Letters of Credit | PPA Company V
Operating Leased Assets [Line Items]
Maximum borrowing capacity				$ 6,200,000	$ 6,400,000
Amount outstanding	$ 5,000,000		$ 5,000,000