Significant Accounting Policies - Narrative (Details)	3 Months Ended
	Mar. 31, 2020 USD ($) FICO_score renewal_option	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Accrued expenses for inventory purchases	$ 3,400,000		$ 15,200,000
SREC inventory	115,107,000		43,749,000
Deferred revenue	68,557,000		58,853,000	$ 34,000,000.0
Revenue recognized	$ 997,000	$ 653,000
PPA revenue
Disaggregation of Revenue [Line Items]
Agreement term	25 years
Renewal term	5 years
Number of options to renew term | renewal_option	2
Lease revenue
Disaggregation of Revenue [Line Items]
Agreement term	25 years
Renewal term	5 years
Number of options to renew term | renewal_option	2
Remittances of customer payments, period after placed in service date	3 years
Solar renewable energy certificate revenue
Disaggregation of Revenue [Line Items]
Typical period for receiving payment	1 month
Loan revenue
Disaggregation of Revenue [Line Items]
Deferred revenue	$ 55,406,000		46,958,000
Other revenue
Disaggregation of Revenue [Line Items]
Agreement term	5 years
Minimum
Disaggregation of Revenue [Line Items]
Agreement term	10 years
Minimum | Loan revenue
Disaggregation of Revenue [Line Items]
Agreement term	10 years
Minimum FICO score required for customer to qualify for program | FICO_score	650
Maximum
Disaggregation of Revenue [Line Items]
Agreement term	25 years
Maximum | PPA revenue
Disaggregation of Revenue [Line Items]
Renewal term	10 years
Maximum | Lease revenue
Disaggregation of Revenue [Line Items]
Renewal term	10 years
Maximum | Loan revenue
Disaggregation of Revenue [Line Items]
Agreement term	25 years
Minimum FICO score required for customer to qualify for program | FICO_score	720
Solar Renewable Energy Certificates
Disaggregation of Revenue [Line Items]
SREC inventory	$ 0		$ 0