Stockholders' Equity - Scheduled Vesting for Outstanding Restricted Stock Units (Details) - Restricted Stock Units - shares shares in Millions	Sep. 30, 2021	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
2021 (in shares)	1.8
2022 (in shares)	5.5
2023 (in shares)	5.2
2024 (in shares)	2.1
2025 (in shares)	0.7
Thereafter (in shares)	0.3
Total (in shares)	15.6	15.2