Stockholders' Equity - Scheduled Vesting for Outstanding Restricted Stock Units (Details) - Restricted Stock Units - shares shares in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Scheduled vesting — restricted stock units
2022 (in shares)	5.4
2023 (in shares)	5.2
2024 (in shares)	2.2
2025 (in shares)	0.9
2026 (in shares)	0.1
Thereafter (in shares)	0.2
Total (in shares)	14.0	15.2	14.3	15.9