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Stockholders' Equity - Schedule of Common Stock Outstanding (Details) - shares
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Increase (Decrease) in Stockholders' Equity    
Beginning balance 536,669,722 461,004,742
Ending balance 537,015,753 468,616,540
Stock Incentive Plan    
Increase (Decrease) in Stockholders' Equity    
Stock issued during period (in shares) 346,031 111,798
Follow-On Offerings | March 2021 Forward Sales Agreement    
Increase (Decrease) in Stockholders' Equity    
Additional shares subject to forward sale agreement (in shares) 69,000,000  
Follow-On Offerings | June 2020 Forward Sale Agreement    
Increase (Decrease) in Stockholders' Equity    
Additional shares subject to forward sale agreement (in shares) 26,900,000  
ATM Stock Offering Program    
Increase (Decrease) in Stockholders' Equity    
Stock issued during period (in shares) 0 7,500,000