Stock-Based Compensation - Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Stock based compensation expense
Total stock-based compensation expense, before income taxes	$ 23	$ 24	$ 27
Income tax benefit	(6)	(6)	(7)
Total stock-based compensation expense, net of income taxes	17	18	20
Unrecognized compensation cost for all stock-based compensation awards	$ 51
Weighted average period for recognition of unrecognized compensation cost for all stock-based compensation awards	1 year 9 months 18 days
Tax benefit from exercise of stock options and other awards	$ 5
Selling, general and administrative expense
Stock based compensation expense
Total stock-based compensation expense, before income taxes	19	20	23
System operations expense
Stock based compensation expense
Total stock-based compensation expense, before income taxes	4	4	4
Long-Term Incentive Plans | Restricted Stock Units
Stock based compensation expense
Total stock-based compensation expense, before income taxes	20	18	16
Long-Term Incentive Plans | Performance Shares
Stock based compensation expense
Total stock-based compensation expense, before income taxes	2	5	10
Non-Employee Directors' Plan
Stock based compensation expense
Total stock-based compensation expense, before income taxes	$ 1	$ 1	$ 1