Stock-Based Compensation - Expense (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Stock based compensation expense
Total stock-based compensation expense, before income taxes	$ 27	$ 32	$ 41
Income tax benefit	(7)	(8)	(10)
Total stock-based compensation expense, net of income taxes	20	24	31
Unrecognized compensation cost for all stock-based compensation awards	$ 41
Weighted average period for recognition of unrecognized compensation cost for all stock-based compensation awards	2 years 3 months 18 days
Tax benefit from exercise of stock options and other awards	$ 11
Selling, general and administrative expense
Stock based compensation expense
Total stock-based compensation expense, before income taxes	23	28	36
System operations expense
Stock based compensation expense
Total stock-based compensation expense, before income taxes	4	4	5
Long-Term Incentive Plans \| Restricted Stock Units
Stock based compensation expense
Total stock-based compensation expense, before income taxes	16	21	22
Long-Term Incentive Plans \| Performance Shares
Stock based compensation expense
Total stock-based compensation expense, before income taxes	10	10	18
Non-Employee Directors' Plan
Stock based compensation expense
Total stock-based compensation expense, before income taxes	$ 1	$ 1	$ 1

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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