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Cover Page - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Mar. 02, 2021
Jun. 30, 2020
Class of Stock [Line Items]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2020    
Document Transition Report false    
Entity File Number 001-36559    
Entity Registrant Name Spark Energy, Inc.    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 46-5453215    
Entity Address, Address Line One 12140 Wickchester Ln    
Entity Address, Address Line Two Suite 100    
Entity Address, City or Town Houston    
Entity Address, State or Province TX    
Entity Address, Postal Zip Code 77079    
City Area Code 713    
Local Phone Number 600-2600    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 84
Amendment Flag false    
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY    
Entity Central Index Key 0001606268    
Current Fiscal Year End Date --12-31    
Documents Incorporated by Reference
DOCUMENTS INCORPORATED BY REFERENCE

Portions of the registrant's definitive Proxy Statement in connection with the 2021 Annual Meeting of Stockholders are incorporated by reference into Part III of this Form 10-K.
    
ICFR Auditor Attestation Flag true    
Common Class A      
Class of Stock [Line Items]      
Title of 12(b) Security Class A common stock, par value $0.01 per share    
Trading Symbol SPKE    
Security Exchange Name NASDAQ    
Entity Common Stock, Shares Outstanding   14,627,284  
Common Class B      
Class of Stock [Line Items]      
Entity Common Stock, Shares Outstanding   20,800,000  
Series A Preferred Stock      
Class of Stock [Line Items]      
Title of 12(b) Security 8.75% Series A Fixed-to-Floating RateCumulative Redeemable Perpetual Preferred Stock, par value $0.01 per share    
Trading Symbol SPKEP    
Security Exchange Name NASDAQ    
Entity Common Stock, Shares Outstanding   3,567,543