Stockholders' Equity - Changes in Common Shares and Treasury Stock (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2018
Class of Stock [Line Items]
Common stock, shares authorized	6,000,000,000	6,000,000,000
Preferred stock, shares authorized	1,000,000,000
Stock repurchase program, remaining authorized repurchase amount	$ 879
December 2018 Share Repurchase Program
Class of Stock [Line Items]
Stock repurchase program, authorized amount			$ 2,000