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Preferred and Common Stockholders' Equity - Common Shares Issued and Outstanding (Details) - shares
shares in Millions
6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Common shares issued and outstanding    
Beginning common stock issued 75.4 74.5
Beginning common stock outstanding 75.4 74.5
Common stock issued upon exercise of options, vesting of restricted stock units and purchases under employee stock purchase plan 2.3 0.2
Common stock issued at end of period 77.7 74.7
Common stock outstanding at end of period 77.7 74.7